Related: there are zones off the coast of Belgium and France that have been used as ammunition dumps after the 1st world war. There's one such zone (https://en.wikipedia.org/wiki/Paardenmarkt) that is close to a very busy shipping route and whose existence was forgotten until the 70s, when people realized that they had 10000 tons of unexploded chemical ammunition lying around just off the coast. In practice there's a thick mud layer on top of the dumped ammo which keeps things stable, but if e.g. a ship were to get stranded, this could cause extensive environmental damage...