'system') is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula with passages to the north and west of the city of Tulum.

As of January 2023[update], it is the second longest underwater cave system in the world, only surpassed by Sistema Ox Bel Ha.

[6] On December 9, 2004, after a dive with two other teammates, Kent Hirsch and Michael Nast were drowned deep in the cave as they got lost and exhausted their oxygen supply.

[8] The connection was found by the Gran Maya Aquifer Project (GAM), led by the cave diver and explorer Robbie Schmittner.

In March 2008, three members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava, explored a section of Sistema Aktun Hu known as the Hoyo Negro pit.