You may know a lot about black holes, but have you ever heard of mysterious blue holes? These are deep underwater sinkholes filled with dark blue water. A new study sheds light on the Earth’s deepest blue hole, the bottom of which remains uncharted to this date.

The study authors claim that the Taam Ja’ Blue Hole (TJBH) located within Mexico’s Chetumal Bay, which was once believed to be the second deepest blue hole measuring 274 meters (~900 feet) in depth, is actually 423.6 meters (~1,390 feet) deep. 

This makes TJBH the deepest blue hole in the world, leaving behind the 301 meters (990 feet) deep Sansha Yonga Blue Hole of the South China Sea. However, the currently measured depth is not final, as no instrument has been able to reach the bottom of this underwater sinkhole.

“The maximum depth is a complete mystery,” Dr. Juan Carlos Alcérreca Huerta, first author of the study and a coastal engineering researcher at  ECOSUR (El Colegio de la Frontera Sur) in Mexico, told ZME Science.

The history and mystery of Taam Ja’ Blue Hole 

Blue holes are typically found in tropical and subtropical coastal regions with bedrock made of soluble limestone or carbonate rock. Water on the surface slowly dissolves these soluble minerals, causing fractures in the rocks at the bottom. 

As more and more water flows through these cracks, the rocks become weak and eventually collapse, leading to the formation of deep underwater sinkholes with caves and tunnels. This process is called karstification. 

“The formation of the TJBH is also completely linked to karstification processes encompassing dissolution and rock fracturing, which have been influenced by tectonic activity and historical fluctuations in sea levels across various glacial periods,” Juan Carlos said.

However, nobody knows when TJBH was formed or how old this blue hole is. While scientists described it for the first time in 2021, local fishermen have been aware of its presence for several decades. 

Interestingly, the depth of this blue hole has been a mystery since its discovery. For instance, initial measurements suggested that TJBH was only 30 meters below sea level (mbsl), but subsequent explorations revealed it to be deeper than 130 mbsl. 

In September 2021, Juan Carlos and his colleagues employed an advanced echo-sounding method to measure its depth again. This approach involved the use of echosounders, devices that emit sound pulses and calculate the depth of an underwater sinkhole by measuring the time sound waves take to reach its bottom and return. 

However, these devices also failed to compute TJBH’s depth beyond 274.4 mbsl because of unusual variations in its water density, salinity, and shape.

Going deeper into the deepest blue hole

In December 2023, the researchers decided to go on another scuba diving expedition to study TJBH. This time instead of echosounders, they took a CTD (conductivity, temperature, and depth) profilers with them. 

This device uses a long cable. One end of the cable reaches the bottom of a blue hole and the other end is connected to a computer on a ship. The cable sends real-time information related to the different properties of water. Scientists further use this data to calculate the depth of the blue hole and study its internal environment.

“On December 6th and 13th, 2023, measurement of new CTD profiles was conducted within the TJBH. The depths attained from the CTD casts on these dates, recorded 416.0 and 423.6 mbsl, respectively,” the researchers note.

However, the researchers didn’t lower the CTD cable beyond 423.6 meters because the instrument could be damaged due to high water pressure. So while the CTD profiler confirmed THBH as the world’s deepest blue hole, it also failed to reach the bottom.

The data from CTD also suggests that TJBH may have a highly complex vertical cave system, with conduits, and chambers connecting it to the Caribbean Sea. However, further research is required to confirm these possibilities.

When we asked Juan Carlos, how his team will now measure the depth of TJBH past 423 meters, he replied: 

“Further exploration of the TJBH’s depths could be facilitated through the deployment of Remotely Operated Vehicles (ROVs) and CTD profilers capable of withstanding the water pressure.”

Why it’s even important to study blue holes? 

Blue holes like TJBH are not just dark deep caverns filled with water. They hold a great amount of information related to aquatic ecosystems. 

“These geological formations serve as “windows” to several research areas offering insights into geology, tectonics, and geohydrological processes in karst environments,” Juan Carlos said. 

For instance, recent findings described an increase in temperature for water depths beyond 400 mbsl. This potentially indicates thermal activity inside TJBH. Additionally, apart from the caves and tunnels, the increase of salinity with depth also suggests its subterranean connections with the Caribbean Sea.

Moreover, TJBH is not devoid of life. It is home to various species of microorganisms, some of which may be unknown to us. For example, the researchers have observed biofilms attached to its walls and mucoid filaments floating within its water column. 

“It is possible that additional microorganisms may exist under low or null light penetration, anoxic and high water pressure conditions at deeper layers within the TJBH, although further investigation is required to confirm this hypothesis,” Juan Carlos told ZME Science.

However, macro fauna is absent inside TJBH likely due to the limited oxygen content, according to the researchers. This means that life inside the world’s deepest blue hole is limited to micro-size organisms. 

The study is published in the journal Frontiers in Marine Science.