This does not sound hypothetical to me. There's several ways you can die doing this:

If this tube is iron and was sealed airtight, the rust that has occurred inside will consume the oxygen within. People have died entering pontoons, tanks, pipes etc for this reason.

If there's decaying material oxygen can be reduced in a similar way, and hydrogen sulfide can also be present, which can incapacitate you even if there's enough oxygen.

It might be okay, it might not. Mostly low oxygen conditions won't give you a warning, you will just pass out.

You should stay out of it. If you can't resist, then rent a gas sniffer to check O2, CO2 and hydrogen sulfide levels.