Course:EOSC311/2023/ Risks and Injuries Associated with Diving in Blue Holes

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Figure 1. Dean's Blue Hole located in the Bahamas

Over the past decade, there has been a growing popularity for recreational scuba diving among both children and adults around the world.[1] Scuba diving requires breathing equipment which supports longer and more sustained experiences underwater. As compared to breath-hold diving, scuba divers can expand their reach underwater, allowing them to gain access to more diving areas across the world. Out of the numerous diving sites, blue holes are one of the most famous traveling destinations which have attracted divers across the world due to their unique appearance and diving experience.

Geology and Blue Hole Diving

Not only are blue holes popular diving sites, but they are also known for having uncommon geological composition and structures. Various geological events have contributed to the formation of these huge caverns in the ocean. However, it is also the distinct geological features of blue holes that give rise to many risks associated with diving in one. The structure of the blue hole also affects and causes different physiological responses within the diver and the effect of some can place the diver in danger. Blue hole diving has been renown for causing numerous injuries, illnesses and even death among recreational divers with a varying range of experiences.[2] However, relevant resources or guidance on diving the blue hole are still relatively scarce. As a result, the effect of geology on risks and injuries associated with blue hole diving is a particular interest of this project. Furthermore, by incorporating the geological analysis of the blue hole, we will also be able to formulate the corresponding prevention strategies and treatment methods.

Geological Analysis of Blue Holes

  •  Blue holes around the world: Blue holes, also known as marine sinkholes, are the approximately circular, steep walled depressions that can be seen on the surface of the ocean and are generally thought to have formed due to the collapse of the surface layer[3]. These huge water-filled caverns are named based on their dark blue appearance and their underwater structure [4]. While most of the existing blue holes are found in the Bahamas, there are also many other blue holes around the world that are known for their distinct features[5]. The deepest blue hole ever discovered is the Dragon’s Hole located in the South China Sea, reaching up to 300 meter in depth. If considering both depth and width, the largest blue hole is the Great Blue Hole in Belize, with a depth of 125 meters and a width of 300 meters.
Figure 2. Formation of a sinkhole
  • Formation of blue holes: Blue holes are known to have formed in the past ice ages of the earth’s history[6]. During the ice ages, the sea levels were much lower than they are today due to glaciation. The formation of massive glaciers locked up much of the ocean water, causing the sea level to drop by nearly 122 meters[6]. During this time, the limestone terrains that were once exposed on the earth’s surface were influenced by chemical weathering. Precipitation can cause rainwater to percolate through the ground while causing dissolution among carbonated rocks such as limestone. Dissolution of limestone and other minerals continued and created cavities and passages underground. In addition, the larger and the more spacious the cavities were, the faster they enlarged due to the greater capacity to carry more abundant flow of water[7]. Consequently, the chemical weathering was so significant and had caused enough limestone to be dissolved away that the support near the surface collapsed. As shown in image to the right, this process creates a sinkhole,and it is also known as a karst landscape [7]. Furthermore, chemical weathering also caused the internal collapse of some of the smaller caverns inside a sinkhole. The arch located under the Dahab Blue Hole in Egypt, for example, was originally the mouth of a cavern in the sinkhole and the arch was the remain after a collapse [8]. Eventually, after an ice age has ended and as water was released due to deglaciation, the sea levels on earth rose again, drowning these sinkholes, and forming blue holes in the end.
  • Why do divers dive in blue holes: Nowadays blue holes have become popular sites for tourists and recreational divers. However blue hole diving can be accompanied by risks.The Dahab Blue Hole located in the Red Sea of Egypt for example, was notoriously known as the “Diver’s Cemetery” and has caused over 100 deaths over the past few decades[8]. Despite various risks, blue holes attract countless diving enthusiasts around the world, simply because of the unique diving experience. There are several reasons why blue holes are favored by scuba divers. First of all, some blue holes are surrounded by beautiful coral reefs. The Great Blue Hole in Belize, for example, is renowned for being home to a variety of exotic fish species [9]. The clear ocean water inside the blue hole also provides divers great visibility, creating a striking view of the coral reefs and marine wildlife. Secondly, due to its vertical structure, there is usually a lack of water current and drastic changes in water temperature in deeper areas of the blue hole, which creates an ideal environment for scuba diving[9].

Physiology of Diving in Hyperbaric Environment

Figure 3. Below the sea level, as the depth increases, pressure increases.
  • During scuba diving, the diver is situated in a hyperbaric environment, which means the pressure due to immersion exert additional physiological stress on the human body as compared to at sea level[10]. This is important as pressure changes underwater are often the direct cause of many illnesses and injuries associated with diving[11]. To understand this, there are two fundamental gas laws that needs to be clarified: the Boyle’s Law and Henry’s Law.
  • Boyle's Law: Boyle’s Law can be expressed using the equation: (P1V1 = P2V2)[12] . The law states that as temperature remains constant and as the pressure on gas varies, the changes in gas volume are inversely proportional to it. To apply this in a diving situation, during the diver’s descent, the pressure applied by water increases and the volume of the gases enclosed in the human body decreases. The opposite is true during ascent. As the water pressure decreases, the gas volume increases[13]. In addition, the atmospheric pressure at sea level is equal to 1atm, and the atmospheric pressure increases by 1atm for every 10 meters that the diver progresses in depth[2].
  • Henry’s Law: The second law to be examined is Henry’s Law. The law states that as temperature remains constant, changes in the ambient pressure are directly proportional to the quantity of gas that can dissolve in a solution. In terms of diving, more inert gasses can dissolve in the bloodstream as the atmospheric pressure increases during descent, while less are dissolved in the blood due to depressurization during ascent[13]. However, it is mainly problematic during ascent, as gasses precipitate out of the blood stream more quickly than the body can clear them[13]. The consequences of this will be explained in more detail in the next section.
  • Physiological response during scuba diving: During diving, the body must adjust itself to adapt to the pressure changes, and the physiological changes vary in different aspects of the body. Normally, high levels of pressure can be tolerated by the respiratory system as long as changes in the concentration of different gasses in the lung are balanced [12]. However, during scuba diving, carrying gas tanks and other diving equipment’s can cause fatigue in addition to the increase in air density due to the depth. This can put strain on the respiratory system and causes breathing to be more effort demanding[12]. In terms of the cardiovascular system, due to immersion, the cold environment causes blood vessels to constrict, and the increased atmospheric pressure also leads to an increase in venous return. Together, the cardiac output increases and blood pressure rises as a result [12].

Risks and Injuries Associated with Blue Hole Diving

  • Risks associated with diving in marine sinkholes: In general, there are four main causes of diving casualties: depth and pressure, the underwater environment, damage to or the failure of diving equipment and usage of inappropriate gasses[14]. In terms of blue hole diving, however, most of the diving injuries and illnesses will be focused on depth and pressure.
  • Decompression Illnesses (DCI): Decompression Illness refers to both decompression sickness and arterial gas embolism. Both will be discussed in detail in this section.
    • Decompression sickness: Decompression sickness (DCS) is caused by the formation of inert gas bubbles (mainly nitrogen) in the blood stream [2]. This is closely related to the depth of diving. According to Henry’s Law, more nitrogen is breathed in and dissolved into the blood when the diver dives down due to pressurization. However, if the diver swims up too quickly and the atmospheric pressure drastically decreases, inert gasses can be released from the bloodstream as air bubbles[12]. Some of these nitrogen bubbles can cause many adverse effects depending on where they travel to. In terms of musculoskeletal DCS, for example, nitrogen bubbles can lead to symptoms such as joint pain, numbness and swelling. If present in the central nervous system, gas bubbles can cause neurological DCS with symptoms including weakness, paralysis, and confusion[15].
    • Arterial Embolism: Arterial Gas Embolism (AGE) is another common cause of diving casualties and is similar to DCS with a difference in the genesis of air bubbles. According to Boyle’s Law, as discussed in the previous section, gasses trapped in the body expands during ascent. Rapid ascent can cause gas bubbles in the lung to be released into arterial circulation. Larger bubbles can potentially block arterioles and symptoms vary depending on the location. If the blockage occurs at one of the blood vessels supplying to the heart or the brain the diver can suffer from a cardiac arrest or a stroke [15].
  • Why is this a concern during blue hole diving? DCS is a major concern when diving in the blue hole. Due to its geological structure and its massive vertical depression, blue hole diving usually requires diving at a greater depth and a longer time. The time exposure to pressure is further increased as some blue holes require divers to swim through interconnecting passages that have formed because of dissolution when the sinkhole originally developed. Take the Great Blue Hole in Belize as an example, the maximal depth that can be reached is around 40 meters and a typical dive lasts around 30 minutes[16]. DCS is a major concern when diving depths exceed 10 to 15 meters and when its length takes above 20 minutes[2]. As a result, exceeding depth-time exposure puts many inexperienced scuba divers at significant risk of DCS when they are not informed of the consequences. On the other hand, as the diver exceeds their diving length and depth limits, the air bubbles formed as a result of DCS may consequently lead to arterial gas embolism during rapid ascent to the surface. Furthermore, AGE is also the leading cause of death among divers [17]. As a result, AGE is also a common illness and a major concern to divers when they are in a blue hole.
  • Nitrogen Narcosis: Nitrogen Narcosis is another common issue during blue hole diving, caused by a high partial pressure of nitrogen in the body. At greater depths, the partial pressure of nitrogen in blood becomes greater when the diver is breathing compressed air[18]. Recreational divers usually breath a mixture of oxygen and nitrogen, and the increase in atmospheric pressure as the diver progresses in depth also increases density of the gasses[8]. Narcosis is mainly caused by the influences of nitrogen on the central nervous system, and the symptoms mimics that of intoxication[18]. This is extremely dangerous at greater depths underwater, as the diver becomes disoriented and makes poor judgements. The diver may continue to swim deeper while thinking that they are approaching the surface and eventually sinking to the bottom due to depletion of gasses[19].
Figure 4. The arch of the Dahab Blue Hole
  • Why is this a concern during blue hole diving? The effect of nitrogen narcosis becomes more evident when the diving depths exceed 30 meters and the risk increases tremendously if the diver reaches below 50 meters [19]. However, in many blue holes, the maximal diving depth reaches 40 meters and below. Take the Dahab Blue Hole as an example, at 56 meters down, the arch that formed as the result of the surface collapse has become a landmark, and many scuba divers set it as a dive objective. Many with moderate to high levels of certification are driven by their ego and attempt to even reach beyond that level [8]. At roughly 50 meters underwater, the symptoms may begin to develop. The diver can become disoriented, and this could cause conflicts among team members. The situation become dire when the team splits in attempt to search or rescue divers affected by narcosis.  Furthermore, the lack of water current due to the vertical structure and the high underwater visibility inside the blue hole also trick divers to underestimate the difficulty of the dive and ignore any abnormalities that appear within the body[8].

Risk Factors and Prevention Strategies

Risk factors and general treatment:

  • DCI: In terms of arterial gas embolism and decompression sickness, the most common risk factors include rapid ascent and exceeding the appropriate depth and length. As discussed previously, these risk factors are the direct cause of injuries via triggering the formation and expansion of gas bubbles in the body. Other known risk factors include being at an older age, obesity, lung diseases and poor cardiovascular fitness[19]. In addition, previous history of DCS or residual defects of a previous DCS also expose the diver to higher risks of decompression illnesses[11] . The most common treatment for decompression illnesses includes recompression therapy, which involves placing the diver in a chamber where high pressure will be applied to shrink the size of the gas bubbles in the body[20]. In the chamber, the diver will be monitored, and most will have an immediate relief of symptoms after recompression [20]. However, most recompression chambers are located in hospitals or diving clinics, which means that divers with urgent DCI are not likely to be sent from the diving site to a chamber in time. As a result, it is more important to prioritize avoiding rapid ascent and long exposure during diving in blue holes.
  • Nitrogen narcosis: The risk factors of nitrogen narcosis often involve the type of gas used, and the depth and length of diving[18]. Fatigue, anxiety, hypothermia, and the use of alcohol can also potentially increase the risk of nitrogen narcosis[18]. The most effective treatment to nitrogen narcosis requires the diver to stay vigilant and be aware of any abnormalities during diving. Effective communication strategies should also be emphasized within the team before setting off. In addition, early recognition is vital when a diver is at greater depths and the diver should slowly ascend before the onset of symptoms[18].

Prevention Strategies:

  • Decompression stops: In the blue hole, depth and pressure are the most prevalent causes of injuries, and limiting the depth of diving is one of the most effective prevention strategies. It is important for recreational divers to restrict their diving depth to within 40 meters and no more than 20 meters among inexperienced divers[14]. Moreover, it is also important to avoid rapid ascent to the surface. It is recommended to ascend at a speed less than 0.15m/sec while also taking a stop for 3-5 minutes at around 15 meters and 5 meters to the surface[21]. This strategy helps divers to prevent DCI via the gradual explusion nitrogen and slowing the expansion of air bubbles. Lastly, in order to minimize the risks of DCI, it is also necessary to avoid flying until 15- 25 hours after diving[21].
  • Follow your route and check your equipment: Before diving in a blue hole, it is crucial to confirm the diving route and check all the diving equipment for every member on the team. Make plans for the entire diving length with your teammates and review the methods for communicating underwater in case of emergencies. Make sure that everyone on the team understands to strictly follow the plans and schedules, as there are high rates of diving casualties even among experienced divers[14]. If the diving depth exceeds 50 meters, such as diving down to or reaching beyond the arch in the Dahab Blue Hole, the diving community generally recommends replacing the diluent gas of oxygen to helium or helium nitrogen to reduce risks of nitrogen narcosis[18].
Figure 5. Use of dive computer in scuba diving
  • Follow your diving devices and dive tables: Dive tables and devices are also useful tools to guide the diving plan. A dive table uses a group of letters in sequence from A to L with each indicating the amount of nitrogen in the body. Within a certain depth, the dive table can be used to determine the suitable length of diving[22]. Another device commonly used by scuba divers is the dive computer. It is a device that takes in account of the depth and time of diving and calculates the time remaining for diving safely. It is favored among divers as it can also inform divers of the time needed for each stop during ascent to prevent DCI[23].
Figure 6. Dive tables (top half of the image) can be also used along with other diving devices to minimize the risk of diving injuries.


Geological processes such as glaciation and chemical weathering had shaped the blue holes into beautiful and beloved diving sites among recreational scuba divers. Nevertheless, it is also their unique geological structure that makes these marine caverns one of the most dangerous dive sites in the world. The enormous vertical depressions and the internal passages contribute to the risks involved in blue hole diving such as depth and pressure. Recreational divers should be educated of the common illnesses and injuries and develop strategies and plans. Divers should also use appropriate devices to avoid decompression sickness, arterial gas embolism and nitrogen narcosis which are the common concerns during blue hole diving. Finally, recreational scuba divers should consult their doctors, kinesiologists and personnel in nearby diving clinics to create their individualized diving plans and become appropriately informed of the risks based on their health conditions.


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