Police in the Marin County town of Ross are trying to get to the bottom of who organized a huge underground pool party last Saturday at an empty mansion that's on the market for $7.5 million. The organizers had rented the house, and gotten security guards and a shuttle bus to take people from a parking lot at the College of Marin up to the house, and via Instagram video it looks like a few hundred people showed up.

The organizers, known as Bump N Tay, threw a similar "mansion party" at a home in the Oakland Hills last July 4th, but this time, the neighbors got a lot more vocal and called the cops multiple times, complaining about noise, and "sex acts" in the college parking lot. Initially, cops were turned away by the security guards who lied and told them it was a wedding of a famous fashion designer. It was actually an event with a cover charge featuring KMEL DJ Rick Lee.

Then an eagle-eyed neighbor found this flyer on the internet and showed police, proving that the organizers were charging $25 admission and selling alcohol with a license, so they shut the party down by evening. But, as an event photographer tells the Marin IJ, the party was still "the place to be that day," and looks like it was fun for all nonetheless.

And at least they didn't just break into the house and throw a party, like those teenagers did in a Novato mansion last summer.

See some video evidence below.

[Marin IJ]