A thirteen-year-old boy is attempting to visit all 270 London Underground stations in one day in memory of his older brother.
Alasdair Clift, from Wirral, Merseyside, took on the "Tube Challenge" to raise money for charity Bloodwise after his brother Adam, 17, died from lymphoma in March.
The teenager began his journey at Chesham station at 5.15am on Monday and expects to finish at Heathrow airport shortly after midnight, having navigated the entire Underground network.
Alasdair, who is "fascinated" by transport, has so far received more than £7,500 in donations - far exceeding his original target of £100.
Richard Clift, 50, who is accompanying his son on the journey, said they were "hot, sweaty and tired" but "having a good time".
He said he had been touched by the "phenomenal" support the challenge had received. "There's a good feeling," Mr Clift said.
"It's the random gestures of complete strangers, for example dashing onto the train at London Bridge, saying: 'You're the guys doing the tube challenge' and immediately giving us £20.
"The staff of Transport for London have been incredibly generous, entering into the spirit of the day. We've had lots of waves and lots of help."
Alasdair was allowed to ride in the front cab for the first two journeys of his challenge and was given a certificate recognising his achievement from Mark Wild, managing director of London Underground, at Southwark station.
Mr Clift said he and his wife Caroline, 51, had been "overawed" by Alasdair, who has single-handedly arranged the challenge.
"We're very proud - and very proud of his organisation skills because Caroline and I have had absolutely nothing to do with it," he said.
"He's done the entire timetable, he's worked it all out. We get to a station and he says we need to be over on platform two or three ... and not only that, he remembers it all."
Donations for Alasdair's Tube Challenge can be made through JustGiving at www.justgiving.com/fundraising/Alasdair-Clift