Longest underwater cave system explored
Sistema Sac Actun
371.958 kilometre(s)
Mexico ()

In January 2018, divers confirmed that the Sistema Sac Actun ("White Cave System") and the Dos Ojos ("Two Eyes") systems located in Mexico's Yucatán Peninsula are connected by a previously unknown channel. As of July 2019, the combined total length of this underwater cave system has now been measured to stretch 371.958 kilometres (231.12 miles), as confirmed by the local Quintana Roo Speleological Survey. In common with current cave naming protocol, when two systems are proved to be joined, the resulting cave takes the name of the biggest of the two previous separate systems.

The previous record holder, Sistema Ox Bel Ha – also located in Mexico's Yucatán Peninsula – contains approximately 270 kilometres (168 miles) of passages.

There are approximately 226 known entrances (sinkholes known locally as "cenotes") to the Sac Actun cave system.