Deepest Mexico: divers in Tulum stumble upon the world's biggest cave system

Mexico's underwater caves were thought holy sites and portals to the gods. Photo / Reinhard Dirscherl, Getty

Dark, underwater caves have intrigued divers and tourists all over the world — and now they have a new one to add to their bucket lists.

A group of divers has just stumbled upon a massive underwater cave just off the coast of Mexico, near the beach resort of Tulum.

At 347km, the cave is the longest in the world, reports news.com.au.

Researchers from the Gran Acuifero Maya (GAM) Project, which studies and preserves subterranean waters off the Yucatán Peninsula, made the discovery after months of exploring watery mazes in the region.

Tulum welcome many tourist divers, but there are serious dangers in the mazy underwater caves. Photo / Reinhard Dirscherl, Getty

They found that two subterranean caves they already knew about — the Sistema Sac Actun, and the Dos Ojos — were actually linked in the middle and together they formed one massive cave system.

GAM director and underwater archaeologist Guillermo de Anda described the discovery as "amazing" and said it could unlock secrets about the Maya civilisation.

At over 300km the underwater cave network of the Yucatán Peninsula is the largest of its kind. Photo / Luis Javier Sandoval, Getty

The Maya people thrived in the region before Spanish conquest in the 1700s.

"It allows us to appreciate much more clearly how the rituals, the pilgrimage sites and ultimately the great pre-Hispanic settlements that we know emerged," Mr de Anda told Reuters.

It is believed the Maya people considered giant sinkholes — or "cenotes" — to be holy sites and portals to the gods.