The hazard
Confined spaces can look harmless from the outside. The air inside can be short on oxygen or hold toxic or flammable gas you may not see or smell, and loose material like sand or grain can bury a worker. When someone goes down, the natural reaction is to rush in and help, and that can turn one victim into two.
The action
Your nose and eyes cannot tell you if the air is safe, so test it before anyone goes in. When testing the air, test for oxygen first, then combustible gases and vapors, then toxic gases and vapors. Ask to see the readings, and if the monitor alarms, get out. (29 CFR 1926.1204(e)(3))
ASK THE CREW
How do we call rescue from this site, and who answers?
From the talk: Confined spaces. Read it in full, free, with the questions and the sign-in sheet.