Ventilation Shafts or 'Vents', are crawlspaces that the player can crawl through to access another area.

Create your vent sector. It should be thin, but still wide enough for the player to fit through. It should also force the player to duck. A ceiling height of 8 units works great. Some good vent textures are (#341), (#342), and (#343).

Place a fan or vent grating (#407, #595, #1113) over the vent entrance. Resize it to fit the gap perfectly. Make it blocked/hittable with the B key, and make sure it's two-sided (use the 1 key if it isn't).