The Bethnal Green tube shelter disaster turned out to be the worst civilian disaster of World War 2, yet very few people know about it.
The Central line had not reached further East than Liverpool Street when the 2nd World war broke out, so work on the unfinished Bethnal Green underground station stopped. It was a natural place for East Londoners to shelter in safety from the bombs, particularly during the Blitz. There was no track and no trains could run through it, so it could accommodate up to 10,000 people sleeping on the platforms, in the tunnels and in the pit where the track would normally have been. Unlike the present 3 entrance/exits to the station there was only one entrance to the shelter during the war. You went through a narrow doorway that opened out straight onto the rough, wide steps, that were open to the elements so there could be no light to show the way down otherwise enemy planes would have seen it.