The Crossrail scheme has hit a stumbling block, after a hopper fell over when it was loading mud excavated from the new tunnels onto freight trains. Here's our exclusive story.
Crossrail is the biggest infastructure project in Europe - costing almost £15 billion and connecting the West of London to the East by rail.
But just a few months into the construction of the Crossrail underground tunnels, work has ground to a halt on two of them.
Paddington's mud mountain is a new and unwelcome addition to the landscape. It is slowly getting smaller.
But this huge pile of London clay is evidence of a major hiccup in Europe's largest engineering project.
And because of it, all tunnelling on the Crossrail scheme has come to a standstill.
With the hopper wrecked, the conveyor belts were switched off and the mud slowly began to pile up at Paddington.
Tunnelling began in May with the first machines heading East from Royal Oak near Paddington towards Canary Wharf and Stratford.
One of the tunnels is already more than a kilometre. But the other drill is only 160 metres in.