seafloor

By looking at the changing scratch marks on the seafloor, the researchers discovered an abrupt shift in Britain's icebergs about 18,000 years ago, a time when the planet was gradually warming from a very cold period.

This is a bit like doing an MRI scan of the sediment layers beneath the present-day seafloor, going back millions of years.

It's also a remarkable feat of engineering, that will see segments of the tunnel placed on top of the seafloor, and then joined together.

Next it's a painstakingly complex procedure, lowering the elements 40 metres down into a trench dug out on the seafloor, using underwater cameras and GPS-guided equipment, to line it up with 15mm precision.

Most underwater tunnels - including the 50km Channel Tunnel between the UK and France – burrow through bedrock beneath the seafloor.