Police are investigating the "appalling damage" caused to an ancient monument by off-road vehicles.
The Bronze Age burial mound, which is a feature of Wentwood Forest in Monmouthshire, is around 3,000 to 4,000 years old.
It is a Scheduled Ancient Monument, meaning it is a nationally important site with legal protection.
Rob Davies, site manager for the Woodland Trust, said: "This is appalling. A feature that is around 3,000-4,000 years old has been damaged within a few minutes. This is a Bronze Age burial mound, a Scheduled Ancient Monument, and the damage caused is therefore a criminal offence.
"I will be informing Cadw of the damage and I have already informed the police.
"Could I appeal to anyone who has any information about this incident, or who knows the registration numbers of any vehicles involved to contact the police?
"There has unfortunately been an on-going problem with damage to this and similar features within Wentwood. The Trust has been spending around £1,500 a year to try to keep vehicles way from them, and following this damage, we will be undertaking further work.”
Gwent Police said immediate intervention measures are being introduced to prevent further damage, and has appealed for anyone with information to call them on 101 quoting reference number 2000006156.