Mexico: Cryptic Sunken Underworld of Flooded Caves and Subterranean Rivers Revealed by Scientists

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Cave passage and diver within a section of the Ox Bel Ha cave system HP Hartmann

A cryptic sunken underworld in Mexico's Yucatan Peninsula is home to a previously unknown ecosystem being fuelled by methane, scientists have announced.

The weird, otherworldly flooded caves and subterranean rivers have been found to host lifeforms in the same way as parts of the deep oceans and lakes —providing scientists with a better understanding of how these hidden worlds function.

Researchers from the U.S., Mexico and Europe ventured into the Ox Bel Ha cave network in the north-eastern part of Yucatan through a huge sinkhole. The sinkhole is also where the freshwater supplying the subterranean estuary comes from—rainfall and salt water from the ocean feed into it to form the distinct water layers found there.

For their study, the team analyzed the lifeforms and water layers found in different parts of the caves. Their findings, published in Nature Communications, shows how methane and bacteria that feeds off it forms the basis of the food system.

The methane forms under the jungle floor above the cave system and then migrates downwards. Microbes eat methane in the water and dissolved organic material brought in by the freshwater from the surface. From these microbes, a whole ecosystem of lifeforms, including crustaceans and shrimp, is spawned. One species of shrimp was found to get 21 percent of its nutrition from methane.

"Finding that methane and other forms of mostly invisible dissolved organic matter are the foundation of the food web in these caves explains why cave-adapted animals are able to thrive in the water column in a habitat without visible evidence of food," lead author David Brankovits, from Texas A&M University at Galveston, said in a statement.

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Map of the Ox Bel Ha cave explored by the team. Brankovits et al

The researchers say the processes identified in the cave will help them to better understand what is happening in sunken ecosystems around the globe, including parts of the ocean that are currently being stripped of oxygen—a major concern to marine scientists working in the field.

Deoxygenation is the expansion of oxygen minimum zones in the oceans as a result of carbon dioxide emissions. This increase impacts marine habitats, potentially causing disruption to ecosystems and causing an imbalance in the oceans.

Marine Biologist Tom Iliffe, also from Texas A&M University at Galveston, said: "Providing a model for the basic function of this globally-distributed ecosystem is an important contribution to coastal groundwater ecology and establishes a baseline for evaluating how sea level rise, seaside touristic development and other stressors will impact the viability of these lightless, food-poor systems."