The longest known underwater cave in the world is even longer than we thought.
As of the start of this year, cave divers have mapped 524 kilometers (325.6 miles) of a subterranean water 'web' near Tulum, Mexico, called Sistema Ox Bel Ha – and there's more to go.
The name Ox Bel Ha is derived from the Mayan language and translates to "Three Paths of Water".
It is the second-longest explored cave system in the world, second only to the Mammoth Caves in Kentucky, which run for at least 686 kilometers (426 miles).
The key difference is that the Mammoth Caves are dry, whereas Ox Bel Ha is filled with water, making it much harder to explore.
Related: Underwater Labyrinth Hidden Beneath Mexico Contains a Huge Swathe of Life
The vast cave system was only discovered by Western explorers in 1996, and explorations didn't begin for several years. In the decades since, cave divers have worked to penetrate as much of the underwater labyrinth as possible.
Each time a dive team maps a section, they must carefully regulate their oxygen and buoyancy, laying down a guideline wherever they go.
This connection to the surface gives them a route to the exit, but it also measures the cave's length, and lets divers know what giant caverns, sinkholes, and 'ice palaces' have already been explored.
A few years ago, a team of cave divers exploring Ox Bel Ha made a breakthrough while re-surveying existing lines. They managed to find around 10 kilometers of new passageways, opening the door to an undiscovered area.
"To our surprise it looked like we were the first ones there," the divers wrote for In Depth magazine in 2023.
Since then, the total explored length of the cave has increased from 496.8 kilometers (308.7 miles) to 524 kilometers, according to reports from the conservation organization CINDAQ (El Centro Investigador del Sistema Acuífero de Quintana Roo).
In 2018, Ox Bel Ha was briefly overtaken as the longest explored underwater cave, as cave divers discovered a connection between two other cave systems in Mexico. Some of the chambers contain ancient Maya treasures.
Today, however, Ox Bel Ha has firmly edged back into first place. Perhaps one day, if further explorations go swimmingly, this may become the longest explored cave system in the world – flooded or not.