I recall reading in the papers many years ago oif a fortification that was at sea level on the Kent(?) coast. Once the steel doors had rusted away by the mid 70's, the kids had cleared the sand away, got in and it was pretty much as it was left. Weapons, papers, tables, radios books etc etc. All rusted solid of course by then because while people hadn't be able to get in, the sea water and damp had been able to enter but only to the lower level. Everyone assumed that it'd been cleared and sealed when it was accessed via rusty steel doors, there it all was.