Deepest blue hole in the world discovered, with hidden caves and tunnels believed to be inside

Mexico's Taam Ja' Blue Hole is the deepest known underwater sinkhole in the world, researchers have discovered — and they haven't even reached the bottom yet. 

New measurements indicate the Taam Ja' Blue Hole (TJBH), which sits in Chetumal Bay off the southeast coast of the Yucatan Peninsula, extends at least 1,380 feet (420 meters) below sea level. 

That's 480 feet (146 m) deeper than scientists initially documented when they first discovered the blue hole in 2021, and 390 feet (119 m) deeper than the previous record holder — the 990-foot-deep (301 m) Sansha Yongle Blue Hole, also known as the Dragon Hole, in the South China Sea. 

"On December 6, 2023, a scuba diving expedition was conducted to identify the environmental conditions prevailing at the TJBH," researchers wrote in a study published Monday (April 29) in the journal Frontiers in Marine Science. During the expedition, the researchers took measurements with a conductivity, temperature and depth (CTD) profiler — a device with a set of probes that read and transmit water properties to the surface in real time via a cable. The data revealed that the Taam Ja' blue hole is "the world's deepest known blue hole, with its bottom still not reached," the researchers wrote in the study. 

Related: Colossal cave in Mexico that formed 15 million years ago is even more enormous than we thought 

The profiler also highlighted different layers of water within the blue hole, including a layer below 1,312 feet (400 m) where the temperature and salinity conditions resembled those of the Caribbean Sea and nearby coastal reef lagoons. This suggests the TJBH may be connected to the ocean via a hidden network of tunnels and caves, according to the study.

Blue holes are water-filled vertical caverns, or sinkholes, found in coastal regions where the bedrock is made of soluble material, such as limestone, marble or gypsum. They form when water on the surface percolates through the rock, dissolving minerals and widening cracks, which eventually causes the rock to collapse. Famous examples include Dean's Blue Hole in the Bahamas, the Dahab Blue Hole in Egypt and the Great Blue Hole in Belize.