Tunnel found in Tijuana is so long authorities need oxygen tanks

> In accordance with federal law, U.S. authorities must fill the local side with concrete after tunnels of the kind are discovered.

Apropos of anything else, that's a strangely specific statute. I have seen multiple references in the media, but couldn't find it. The closest I've come is 18 USC 555 on the offenses of allowing illegal border crossing tunnels or passages (https://www.law.cornell.edu/uscode/text/18/555) but not on required remedies for law enforcement.

How long is 6 football fields in Kilometres?

How many bananas long?