During the 1860s two tunnels were constructed almost simultaneously between King's Cross and Farringdon - one carrying a railway and the other burying a river. The river in question was (of course) the Fleet, whose disappearance underground was part of the construction of London's great sewer system (of which more later). And the railway was the world's first underground railway, opened in 1863 between Paddington and Farringdon as part of a grand plan by the MetropolitanRailway to link together several North London rail termini. The line was constructed using the 'cut and cover' method (dig a big trench, line it with bricks, cover it over) and for most of its route followed the Marylebone and Euston Roads, which caused massive traffic chaos during the construction period. Strange but true fact: the earth excavated during the construction of the Metropolitan Railway was dumped on the southwestern outskirts of London at 'Stamford Bridge', where it was later used to create the terraces at Chelsea's new football ground.
On leaving King's Cross the new Metropolitan Railway burrowed in an open cutting (better for letting the steam escape), and if you stand in Wicklow Street (or any of several neighbouring streets) you can still see the gaps carved through the terraced houses where the railway passes below. The tracks then continued southeastward buried beneath the Farringdon Road, and it was here in the summer of 1862 that disaster struck. The new parallel Fleet Sewer suddenly and catastrophically burst its walls, flooding a half-mile stretch of the railway to a depth of ten feet. Here's how the Illustrated London News reported the event (and you can see their front page illustration here).
Oh, and while we're on the subject of the Underground, yes it is true that the Jubilee Line was once going to be called the Fleet Line (because it was originally planned to run beneath Fleet Street). But it's the Metropolitan that's the true Fleet line, and always will be.