Inaccessible chambers in PC TNT level 3, Power Control:

On this level there are 3 tiny chambers that are normally
hidden and sealed shut in single-player mode: One on the
north edge of the map, and two others branching from the
short tunnels leading to the eastern-most and southern-most
areas.

These are normally accessed only in Cooperative games, via
a special set of invisible one-time teleporters, arrayed
just ahead of the first teleporter pad at the start of the
level.