Tretyakov Gallery — Moscow, Russia

The tsar's government wanted this out of sight — it was apparently barred from public display for a while not long after it turned up. Part of what's said to be behind it is the political killing going on in Russia at the time, so it isn't really a history lesson so much as a picture about violence the painter was living through. It's also been attacked by visitors with a blade twice, decades apart.

According to Wikipedia he spent months on more than thirty studies for this, then put the final thing down in about three or four days. The experts apparently read the Flood and Judgment Day into it — though nobody seems to claim you're actually supposed to be able to see them.