IT is so creepy it even gives the hard-headed owners of Clearwell Caves a shiver – a mystery being stalking the old mine workings at night.

"We have a new recording machine that counts people in and out of the Caves constantly just so we don't leave anyone in there when we lock up at night," says Ray Wright, who runs the attraction with his son Jonathan.

"But for some time now it has registered somebody roaming down there occasionally when it's shut, usually between 2.30am and 4am, but not every night.

"It is quite sensitive and is able to differentiate between adults and children...and this is definitely an adult.

"When we open up in the morning there is no sign of anything being disturbed."

Ray said one of their theories was that a crew filming an episode of 'Merlin' in the caves earlier this year had inadvertently left an alien – one of the cult TV series' new characters – down in the tunnels when they left.

Oddly enough, before the TV show 'adopted' a being from space, there were plenty of theories in magic and mystery circles that Merlin himself had extra-terrestrial powers and might even have been a space tourist...

"If we do meet up with any little green men we will let everyone know," says Ray.

"For the moment, it's just a puzzling series of manifestations."