Tube challenger pulls out all the stops in latest record attempt
Geoff Marshall is a man on an Underground mission.
He desperately wants to break the world record for the Tube Challenge - visiting all 270 London Underground stations in the fastest time possible.
This is his 24th attempt, but only once before has he got his name in the record books.
Rules dictate that competitors must arrive or leave from all 270 stations on the network on a tube train, and are allowed to run, or use all forms of public transport to get between stations. However, no forms of private transport are allowed, such as cars, taxis or bicycles.
The current record - 16 hours, 29 minutes and 13 seconds - is considered in Tube Challenge circles to be almost unbeatable, but that did not stop Geoff and his team of helpers setting off on the 250-mile adventure.
Video Journalist: Dan Curtis