Taam Ja
The world's deepest blue hole (marine sinkhole) lies off the coast of Mexico's Yucatán Peninsula. It's at least 420m (1,378ft) deep, but explorers still haven't found its bottom.
Known as 'Taam Ja', or 'deep water' in the Mayan language, it was discovered more than 20 years ago by a local diver, but it wasn't until 2021 that scientists attempted to measure its depth. Using an echo sounder to bounce sound waves off the bottom of the hole, they estimated Taam Ja' was 275m (902ft) deep.
But another expedition in 2023 used a device called a conductivity, temperature and depth (CTD) profiler, with sensors that measure water pressure, to estimate its depth. This returned a reading of 420m (1,378ft), but they're not sure it reached the bottom.
Blue holes are naturally occurring caverns on the sea floor, with vertical walls. They're usually found in coastal regions where the bedrock is made of soft, soluble rock, such as limestone. This is gradually eroded, causing the rock above to collapse. Although usually only a few tens of metres deep, blue holes sometimes connect to underwater cave systems.
With poor water circulation, blue holes are low in oxygen deep down. This makes it hard for organisms to survive, but some microbes thrive there, consuming sulphur-based compounds, such as hydrogen sulphide. Other small creatures - including shelled single-celled organisms called foraminifera and tiny worm-like creatures called nematodes - have also been found living in blue holes. Scientists are keen to explore Taam Ja' to find out what creatures live in its dark blue waters.
