You wrench the trapdoor open, strings of gelatinous glue slithering over your fingers. A blast of cold air rushes in - this is clearly a shaft to the vents. You squeeze into the tiny gap, the darkness and the metal walls pressing in upon you, and you climb the metal rungs riveted to the side.
You reach the main vent system. A stream of air-conditioned air gusts past, making you shiver. Iron vent hatches guard either end of the crawlspace.
You are becoming cold, and need to make a decision quickly. Which direction do you crawl in, against the air flow, or the other direction?