Forget the Amazon; forget India's forests with their living bridges; even forget the remote Malaysian rainforest with one of the world's tallest trees. The world’s oldest forest is in Cairo. No, not Cairo the capital of Egypt – Cairo, the small town in upstate New York.

At the bottom of a sandstone quarry in the small town of Cairo in New York state, researchers found a network of trees that they think may have once spread around 400 kilometers (250 miles). The extensive network is thought to be 386 million years old, making it the oldest forest in the world. 

"You are walking through the roots of ancient trees," Dr Christopher Berry, a paleobotanist at Cardiff University, told Science in 2019. "Standing on the quarry surface we can reconstruct the living forest around us in our imagination."

The forest was discovered in 2019, as researchers mapped over 3,000 square meters (32,000 square feet) of the Devonian period forest in the foothills of the Catskill Mountains in the Hudson Valley. They found that the forest was home to two types of trees: early fern-like plants called cladoxylopsids; and Archaeopteris, trees that had a woody trunk and flattened green leaves coming out from frond-like branches. Scientists also think they could have uncovered a third tree species in the area that would reproduce, like these others, using spores rather than seeds.

"It is surprising to see plants which were previously thought to have had mutually exclusive habitat preferences growing together on the ancient Catskill delta," said Dr Berry in a press release about the discovery.

The roots of the Archaeopteris trees were found in one area to be over 11 meters (36 feet) in length. These are some of the first examples of complex roots systems that grew as the plants grew, with many branching sections. Prior to this, plant roots were unbranched, and died off and were replaced as the plant grew larger, the authors explain in the paper. 

"This would have looked like a fairly open forest with small to moderate sized coniferous-looking trees with individual and clumped tree-fern like plants of possibly smaller size growing between them," continued Dr Berry. 

Many fish fossils were also recovered from the site, leading the team to believe that the once enormous forest could have been wiped out via flooding. 

The previously believed oldest forest was the Gilboa Forest, only about 40 kilometers away (25 miles) from the new record holder, but it is thought to be around 2 or 3 million years younger than the one found in Cairo. In 2016, a huge tropical fossil forest was discovered in Norway by the same team.

The rise of larger trees with more complicated roots systems is thought to have also triggered a process called “weathering”, during which carbon dioxide is pulled from the atmosphere and eventually stored as limestone. With the drop in carbon dioxide levels, oxygen levels were able to increase, leading to larger animals and insects evolving within the ancient forests. 

“In order to really understand how trees began to draw down carbon dioxide from the atmosphere, we need to understand the ecology and habitats of the very earliest forests, and their rooting systems,” said Dr Berry.