With the help of a local fisherman, identified as Jesus Artemio Poot Villa, scientists found the second-deepest blue hole in the world in an unlikely place — the shallow Chetumal Bay located on the southeastern side of the Yucatan Peninsula.

Filled with a hostile, oxygen-starved environment, the newly-named Taam Ja’ Blue Hole (TJBH) could one day invite research into how life could survive on alien planets or other harsh environments.

What Is a Blue Hole?

A blue hole is a vertical cave typically carved out of eroded limestone that opens onto the ocean floor with a rounded mouth, as is the case with TJBH. The deepest to date — the Dragon Hole in the South China Sea — measures 987 feet, which makes the new Chetumal Bay find the second deepest blue hole at about 900 feet, nearly the length of the Eiffel Tower. The Caribbean has plenty of other, shallower blue holes, too, including Dean’s Blue Hole, now the third deepest at 663 feet deep.

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How Blue Holes Form

The researchers from Mexico propose that TJBH formed from flooding and erosion during the Last Ice Age, given its dramatic depth. To explore it, they ventured forth with both scuba equipment and a submarine, diving all the way down to the lowest, most oxygen-starved layer of the hole to take water samples. They found the waters there to be cold, slightly acidic and almost completely devoid of oxygen, thanks to the lack of photosynthesis.

At such “anoxic” layers, only microbes specially suited to living an oxygen-free life can survive, including those that metabolize sulfates, chemicals toxic to most other life. Extraterrestrial microbes with limited access to oxygen may produce energy along similar pathways.

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What Is the Hole in the Ocean Made Of?

Along the walls of this ocean sinkhole, the researchers found rocky ledges covered in sediments and biofilm, a slimy layer of bacteria. Meter-long “mucoid filaments” floated freely through the water, and small worms and dead barnacles covered the limestone. Such filaments have been found in other oxygen-deprived blue holes.

Higher up, in the water column, the researchers found a layer of “detrital particles” collected from the surrounding ocean, like an aquatic garbage pail.

The study identified three layers in the hole overall: a salty hypoxic layer at the top with slightly reduced oxygen content; a chemocline layer in the middle laced with ammonium, sulfates, sodium and other chemicals; and lastly, the oxygen-starved anoxic layer at the bottom. Scientists found similar stratification at the Dragon Hole and the Amberjack Blue Hole in the Gulf of Mexico.

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Ocean Sinkhole Pools

TJBH sits close to a group of ocean sinkhole pools filled with water locals call “pozas” (pools), and the new paper suggests classifying them as blue holes, too. Their limestone shape suggests that they formed through similar means: roughly circular, steep-walled and extending deep below sea level.

The interaction of limestone and water has wrought other wonders in the area, including the famous cenote sinkholes formed when an upper layer of the rock collapses, exposing groundwater. The Mayans used cenotes as a water supply and, occasionally, for ritual purposes.

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