Nuclear bombs are seen as weapons of mass destruction, but some scientists imagined their use for good. Controlled explosions could be used to divert rivers, dig reservoirs, drill for oil and even create natural harbours. Located in Kazakhstan, Lake Chagan was one of these tests, performed by the Soviet Union in 1965. A 408m wide, 100m deep crater created with a 140 kiloton nuclear explosion. But would you really want to swim in it?
While still radioactive, Lake Chagan is not as bad as it once was as the radioactive fallout decays. Drinking the water would be a bad idea - radiation levels are about 100 times the permitted level in the water. But for swimming the dose would be low enough not to cause any long term issues. 100 meters away from the lake, the radiation level drops back to background levels.
So how many of these sort of projects were tested? In the USA this sort of research was dubbed Operation Plowshare and had 27 nuclear explosions up till 1977. Of course the the Soviet Union had their own much larger program (of which Lake Chagan was part) - testing up until 1989 with 156 nuclear explosions. Luckily for us they even filmed the explosion - skip to 1:55 for the boom.