shift-overbuilding Nuclear reactor leaves a crater and allows overlapping (b566)

So the middle segments become invincible and won't collapse if they have other segments behind them?

yes, as does the original building. i believe then once those get into a bugged state you can sometimes dig under or dump over the end extension, this allows the entire depot to float in the air and likely allows it to be buried too

try it in sandbox

place a depot with some extensions then mine under the middle extension

that's fun - maybe a train depot could be hidden inside a mountain