That was more of a side effect from them fiddling with things they don't understand. It seemed easier to teach them a lesson, and Shigaraki wanted to make use of it.

But that lab went out of use a while ago. It's not part of our projects.

[The tunnel they enter is just tall enough for a couple of people to fit comfortably without banging their shoulders or hitting their heads, an unnaturally neat aperture in the rock face. It doesn't look cut out, but rather like something simply pushed aside the rock as though it were in the way.

Once they get a little ways in, Billy will see why. Overhaul stops to strip his gloves and place his hands on both walls, and if he were to look behind him -- he would see the entire tunnel closing up as though it were never there, stopping just short of where they are. It's a pretty decent ways into the underside of the island by then.

Overhaul nods at him.]

A precaution. I doubt they'll find us either way, but it cuts down on more incidents.

[Then he turns and moves deeper into the underground. They'll eventually reach a metal-lined hall more like what you'd expect from an underground base, which also very eventually opens up into a bigger metal room that looks very much like a lab, though it also has what looks like a reception area with plants (real??) and a coffee table and couches.

There's tea set out, and there's Shigaraki as well.]