Today Ruislip is a quiet West London suburb. Fifty-six million years ago, however, it was subtropical wooded marsh that formed part of the island's coast.

While working on excavating the area for a high-speed railway, experts uncovered evidence revealing the area used to be the swampy, prehistoric coastline of Britain, reports Nicola Davis for The Guardian.

The discovery was made by a team of experts including Jacqueline Skipper, a geological specialist at the Geotechnical Consulting Group, which is assisting with the excavation for a planned high-speed railway in the U.K. called High Speed 2. The team took samples from about 108 feet below the surface of Ruislip and found a previously unknown black clay-like material that formed 56 million years ago, suggesting a coastline from the late Paleocene period.

Skipper tells Davis: “When we looked at it in detail, instead of the usual sand and gravel ... we had a black clay, which not only had bits of vegetation in it but also showed evidence of extreme weathering of what would have been sand and gravel there before.”

She adds: “Suddenly you have got evidence that this is actually the coastline.”

As a video released by HS2 explains, experts have used ground-penetrating radar and drilling to explore material beneath 8,000 locations between London and West Midlands, where the first phase of HS2 will run.

Those ground investigations began in 2015. More than one million lab tests have been completed on ground samples, according to a press release.

The black clay material was found in 2017 in several different locations along a six-mile stretch of the proposed route and has been since undergoing examination.

Skipper and her team believes the material, a layer of about 3 feet thick, dates back 56 million years ago when the grassy area was covered in trees and swamps and had a hot climate and nearby beaches. Davis reports that samples nearby showed sand and gravel at the same depth that were likely deposited by the sea, suggesting the coastline.

At this point in the Paleocene period, dinosaurs were extinct, mammals and birds began to evolve and the Earth was filled with dense forests.

During this time, scientists believe the earth was much warmer—so warm that there were no ice caps and seas were significantly higher than they are now, leading to the coastline so far inland on today's Great Britain. But the discovery of the black clay-like material was a surprise, Skipper tells Davis, because rises in sea level don’t usually leave behind traces of sediments and sea levels continued to rise even after the material formed.

She says: “If you have sea level rise you also have a lot of storms and reworking of the previous sediments, so you don’t always get that much information.”

According to Dave Entwistle, an engineering geologist at the British Geological Survey, this is the first time a deposit of this kind — formed by a marsh in this time period — has been found or identified in England. “The woodland marsh may have only existed for a relatively short time before river deposits covered it,” he tells Davis.