dumped trash

if trash is dumped in the world it should very very slowly decompose into dirt

Or at the very least have it not reset the pollution timer if you move it.

it wouldn't stay flat either -- as it decomposes and reduces in size, anything on top of trash (buildings, rock, slag, etc), will cave in. it's definitely not a surface that can be built on (even if I put another layer of rock over the top).
maybe it'd be funny to add cave-ins to the game, so people who build their docks out of trash can watch it go unstable and collapse over time (I'll post it as a suggestion).

It appears this is in the game now. It does take a really long time though.