Really, a bunch of lame turrets never stood a chance against them. So after they're all systematically destroyed by their efforts, a few of the ruined turrets automatically descend back underground -- but one of the trashed ones is trashed in a way where the wreckage jams up the mechanism that would otherwise allow the base to descend and be swallowed up by the earth again.

They now have a perfect, mostly clear shaft down to what's very obviously a complex underground, complete with conveniently climbable crossbars along the sides.

With Arcade trying to watch multiple simultaneous fights all over the arena (thanks to him amping up the attacks of the Tributes) he's briefly turned away from their fight, instead focusing his attention on their teammates' struggles elsewhere.

Who knows what's down there in the dark? There's only one way to find out.