LONDON, March 16 (Xinhua) -- Engineers working on Britain's biggest ever rail project revealed Friday they have discovered an old coastline showing when London was beside the seaside.

The engineers working on the multi-billion dollar high speed rail link HS2, found the sub-tropical coastline dating back 56 million years at Ruislip in West London.

The layer of black clay they found is thought to have been formed from densely wooded marshes on the edge of what was once a sub-tropical sea.

The rare discovery was made at a site in Ruislip when a HS2 ground investigation team unearthed a previously unknown material, located up to 33 meters below the surface.

The layer of black clay, which HS2 Ltd has named the "Ruislip Bed," is thought to have been formed from densely wooded marshes on the edge of a sub-tropical sea.

The engineers made the surprising finding while investigating ground conditions in the area, prior to the construction of the Northolt Tunnel, a 14 kilometer tunnel for the new rail which will link London with northern England.

Dr Jacqueline Skipper, a geological expert from Geotechnical Consulting Group, said: "Although ground investigations regularly take place across the country, it's really exciting and very unusual to come across a material that no-one has ever seen before. The 'Ruislip Bed' discovery is particularly fascinating, as it is a window into our geological history.

"It would have been formed during the Paleocene period, which was a time of intense change, with new animals evolving following the extinction of the dinosaurs. Most of southern England was covered by a warm sea and this clay helps us to pinpoint where the coastline was."

Steve Reynolds, HS2's ground investigation program manager, said: "We have been investigating ground conditions along the route of HS2, so we can design the railway in the safest and most efficient way."

"We have been exploring the soil and rocks beneath the surface through various methods, ranging from drilling into the ground and taking samples to using radar technology. Our main investigations are almost complete, with over 1 million laboratory tests undertaken on the samples we have taken. It's the largest ground investigation program that the UK has ever seen and an unparalleled opportunity to learn about the ground beneath our feet,"the manager said.