Perhaps the rumours are true, if so maybe for a good reason. Maybe so people dont go in it?
Maybe it is a top secret?
Ive heard it has bad gas/methane no oxygen. and is a part of a sewer overflow/detention tank.
You would need a scuba tank and regulator, or a cfa gimp mask fire mask.
If they are grassed over then all you would need is a cheap metal detector to find them. Idk you might even find some treasure too.
Pretty easy to go and check if you know where it's supposed to be. But as Bam noted entrance isn't recommended without breathing apparatus or access to the control room fans which, at last notice, was alarmed.