Crossrail's excavated earth used to make nature reserve
26 May 2015 Last updated at 06:32 BST
Next month, tunnelling comes to an end on Europe's biggest engineering scheme, the Crossrail project linking rail services to the east and west of London.
But what happens to the millions of tonnes of earth that's been excavated?
Much of it has been shipped to what will be one of the largest nature reserves in Europe, on Wallasea Island off the east coast of England.
The BBC's Andrew Bomford has been finding out more.