An illegal quarry rave went on for 12 hours due to "difficult" terrain and an "intoxicated" crowd, police said.
Avon and Somerset Police was called to Asham Quarry near Frome, Somerset, at about 01:20 BST on Sunday.
About 200 people attended the rave and police seized five van-loads of music equipment as they worked to disperse the crowd. The music stopped at 09:30 and the site was cleared by 14:00.
Thirteen people arrested on suspicion of causing a public nuisance have since been released under investigation.
Police said the rave was well under way when officers arrived, with 200 people and 100 vehicles at the site.
The quarry has a single public highway entry and exit point, heavy foliage and treelines, and zero visibility late at night, police said.
A spokesperson added the terrain made it "difficult" to shut down the event safely "during the hours of darkness" when many in attendance were "intoxicated".
The force asked the public to make themselves aware of the warning signs of illegal raves – including advertising on social media, large crowds gathering, and vans unloading equipment.
A police spokesperson added: "Unlicensed music events of this nature can have a serious impact on communities and we will seek to prosecute organisers, seize equipment and vehicles.
"Avon and Somerset is no place for unlicensed music events and raves which negatively impact or disturb our communities."
