A 125,000-year-old red mangrove forest, renowned for loving saltwater habitats, is mysteriously flourishing on the banks of a freshwater river in the Yucatan Peninsula. Over 200 kilometers (124 miles) from the Gulf of Mexico coast, the ancient forest is very far from home. 

Mangroves are salt-tolerant trees, shrubs, and palms usually found along tropical and subtropical coastlines. However, as Octavio Aburto-Oropeza, co-author of a new study published in PNAS explained in a statement, this particular forest has been “trapped in time for more than 100,000 years,” hence its unusual location.

Stranded inland during the last interglacial period, when sea levels were six to nine meters (20 to 30 feet) higher than now, the ancient coastal ecosystem now finds itself beside the San Pedro Martir River, which runs from Guatemala to Mexico. The team found the forest was home to nearly 100 species that usually prefer saltier climes but had adapted to a freshwater environment some 125,000 years ago, persisting in their beloved home despite the receding ocean. The team speculates that they were able to survive thanks to the calcic waters that replaced the sea – the river runs through limestone, so its water is rich in calcium carbonate.

Using genetic, geologic, and vegetation data, plus sea-level modeling, researchers have shed light on the “lost” forest for the first time, allowing us a glimpse at the global environment of the last interglacial period – a time when Earth warmed and ice caps fully melted. As the planet cooled and sea levels dropped, the mangrove ecosystem quite literally stood its ground and became a relict of a warmer, saltier past. The findings highlight the impact climate change had on the once-coastline, and on the lowlands of the Gulf of Mexico, which were apparently underwater during the ice ages.

The authors hope that their work will serve as an opportunity to better understand the effects of climate change on coastal ecosystems, as sea levels once again rise.

“The San Pedro mangroves are warning us about the dramatic impact that climate change could have on the coastal plains of the Gulf of Mexico if we do not take urgent action to stop the emission of greenhouse gases,” the authors said.

Protecting the region around the study site, largely deforested in the 1970s, is also vital, the authors add. They “hope [their] results convince the government of Tabasco and Mexico’s environmental administration of the need to protect this ecosystem. The story of Pleistocene glacial cycles is written in the DNA of its plants waiting for scientists to decipher it.”