Dirtsurfing is a combination of surfing, biking and mountain boarding. It's great fun but there is a downside - you have to walk back up any hill you surf down!
Simon Watmore sent in this clip of himself (yellow board), Andy Davies (yellow board) and Gavin James (green board) flying down the fire tracks in Llantrissant forest which have been opened up and maintained by a group of local mountain bikers.
Tim Davis filmed it from his own board whilst hanging on to a Sony HD camera!
Andy said "They're really quick runs and lots of fun!"