Ventilation Shafts or "Vents", are crawlspaces that the player can move through to get to another area.
Create your vent sector.
Make it small (but still big enough for the player to fit through). Some good vent textures are #341 to #343.
Place a vent grating (#595) over top of the vent entrance. Resize it to fit over the entrance. Make it blocked by using the B
key, and make sure it's two-sided (use the 1
key to alternate between one-sided and two-sided).