Nothing ventured, nothing explained

You descend the stairs into the basement, followimg the sound. What kind of experiments were they doing here, that needed to be built beneath the Earth? What could need -

FLASH

"Hey Professor Doe! I just thought we should show you the naughty costumes we've got ready for the Halloween party next week," the blonde says to you.. Normally you'd be sorely tempted by an offer like this, but this is the third group this week trying to pre-empt your decision. Still -

FLASH

... what the fuck was that?