Bottom of World's Deepest Blue Hole Proves Elusive

The deepest known ocean sink hole is so deep, researchers using advanced equipment can't find the bottom, though they've found evidence of a whole network of underground tunnels somewhere in its depths. The Taam Ja' Blue Hole in Mexico's Chetumal Bay was formerly considered the second-deepest blue hole, with an estimated depth of 900 feet (274 meters) below sea level, just shy of the reported 987-foot depth (301 meters) of the Sansha Yongle Blue Hole in the South China Sea, per CBS News. But when scuba divers set out to explore Taam Ja' in December, they found water depths surpassing 1,380 feet (420 meters) below sea level. Most remarkable, researchers say the bottom of the hole is "yet to be reached."

The previous depth estimate came from an echo sounder, which calculates the distance sound waves travel. It's not considered super precise in measuring blue holes because fluctuations in water density and a hole's unpredictable shape can mess with the sound waves, Live Science reports. The new measurements come from SWiFT CTD, a package of instruments that measure conductivity, temperature, and depth, but can only operate at depths up to 1,640 feet. Researchers believe the CPT profiler ran into trouble at its maximum depth. According to Live Science, "the cable it was attached to may have drifted on underwater currents or bumped into a ledge that stopped the device in its tracks 1,380 feet down."

The data gained, described in a study published Monday in Frontiers in Marine Science, highlights distinct layers of water within the hole. In one layer, deeper than 1,312 feet, water temperature and salinity climbs to a level similar to the Caribbean Sea and coastal reef lagoons, raising the possibility that Taam Ja' is "part of an underwater intricate and potentially interconnected system of caves and tunnels," according to the study. It's also possible that the warmer water is linked to deep volcanic or tectonic processes yet unknown, Gizmodo reports. Researchers ultimately hope to measure the blue hole's true depth, though that will require "advanced underwater navigation technologies," according to the study. (More discoveries stories.)