Police have said they will have to be more aware of illegal raves in the future after 500 people gathered at one in mid Wales at the weekend.
Hundreds of people gathered near Drovers Arms
Eleven people arrested on suspicion of drugs offences are being held after the event on Ministry of Defence land in the Epynt mountains, near Brecon.
Police and military personnel were called to the event on Sunday.
The rave continued into Monday and Dyfed-Powys Police say more illegal raves could follow.
Officers said they worked with military and local authorities to minimise disruption to the local community.
Chief Insp Huw Meredith said: "It appears that raves are sort of increasing now in popularity again and we'll have to be...aware of the possibility of future events taking place."
Farmers near the illegal rave, near Drovers Arms, said their livestock was disturbed adding that they hope the area has not been damaged.
The Dyfed-Powys force said they will be able to make a more accurate assessment of what damage has been done now the rave is finished.
North Wales Police were called to "small scale gathering" after reports of loud music coming from the quarry in Waunfawr near Caernarfon, Gwynedd on Saturday night.
A police spokesman said they had received "total co-operation" when they visited the site and no arrests were made.