A mountain biker is warning people to be aware after finding a plank of wood with 200 nails embedded in it buried across a forest path.
Chris Williams, from Blaenavon, came across the trap on Monday while out with friends during a ride on the Blorenge mountain in Abergavenny.
He said he was lucky to have spotted the nails poking out of the soil.
"I was in front and I could see something shining," Mr Williams explained.
"It was in a trench and buried. I pulled my brakes on and shouted to the boys."
Mr Williams said he had never seen anything like it before, and described it as "pretty gruesome".
"It's about a metre long, has 200 nails in it it's pretty heavy," he said.
"Whoever made it must have used a nail gun because there are no hammer marks.
"It's also got legs on it, so if you rode over it would not tip up but would stay in the ground.
"Someone has gone to a lot of work."
Mr Williams said it would be irritating for a biker to ride over it, but said it could be much worse if someone fell on it, or if an animal walked on it.
He said the trail had been used for years by walkers and bikers.