Crossrail tunnel
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Tunnel mud used to help birds and wildlife

26 May 2015 Last updated at 16:45 BST

Millions of tonnes of mud that's been dug up to create huge new train tunnels is being used to build a new home for wildlife.

Train company Crossrail are digging out 26 miles of tunnels under London to make a fast train line from east to west.

And the mud left over is being used to create a special salt marsh for birds and Wildlife.

Watch Leah's report to find out more.