Looking for civil or structural engineers

i have a question regarding man made tunnels.

in the video below at the 1 hour and 5 minute mark our explorer enters some sealed of german tunnels from WW2, now my question is this:

if they are sealed off then why is there a very small entrance that he managed to squeeze through, surely they should have been completely closed and not left this small opening? it can’t be for ventilation as no one is supposed to go in there, so could it be to let something out like some sort of radioactivity or natural gas?

- YouTube

It’s pretty cavernous, (not a civil or structural engineer, just interested in nuclear reactors) so on one hand it could be a sort of exhaust for radioactive steam? Like an emergency vent? Doesn’t explain the tiny opening though :thinking:

Sorry, I’m not a civil or structural engineer, but I am a geologist and a caver. If I were to take a guess as to the purpose of the small entrances, I’d say they are for bats. :bat:
https://scienceinpoland.pap.pl/en/news/news%2C86467%2Cover-700-bats-found-hibernating-wwii-bunkers-largest-number-record.html
Numerous bunkers, (both intact and destroyed), abandoned mine shafts and of course caves, serve as bat hibernaculums. Since bats provide valuable and free insect pest control, they are usually a protected species and efforts are made to provide shelter for them. Gates on caves and mines are specifically designed to allow bats free passage and exclude people, who generally tend to disturb and endanger them. At 1:17, the narrator complains that everywhere there are “secrets underground”, the areas are turned into nature preserves. Who lives underground? Bats…

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thanks for the help everyone and I will continue to follow Tino von struckmann’s adventures into Nazi bunkers from world war two and his quest to get more information on the Germans nuclear reactors and atomic weapons program.