It seems crazy that we occasionally hear about unexploded ordnance unearthed in Western Europe but little to nothing east of Berlin when the Eastern Front saw massive attacks and counterattacks.
It's still there. A lot of it was cleaned up, of course, but you periodically see explosives turn up even in Moscow (1, 2, lots more ). I've personally found some on digs. Periodically there are news of "black diggers" being arrested for having a whole bunch of explosives lying around that they dug up.