The flooding which has caused days of misery for rail commuters has been blamed on blocked drains.
Thameslink services through central London have faced severe disruption since the track became submerged last week.
Stomach-churning pictures released by Thames Water show a metal grill by silt and debris.
The flooding, between St Pancras and Farringdon, was caused by a burst water main.
Thames Water said it was initially unclear why the water refused to drain away but an investigation revealed the blockage.
The water in the tunnel cleared as soon as we cleaned out Network Rail’s surface drainage system last night. The pipe was full of silt and debris, and a grill which would have allowed the water to drain away was blocked solid. Basically, the water had nowhere to go. We believe this problem was first identified as far back as 2007, and the problems with water on the track have been caused by a lack of maintenance on their part.