Teams of young people, including one from Guernsey, begin the Ten Tors Challenge
Thousands of young people are taking part in a gruelling two-day trek across Dartmoor.
About 2,400 people, between the ages of 14 and 20, set off for the annual Ten Tors challenge at 0700 BST.
The 400 teams taking part will complete a 35, 45 or 55-mile route (56, 72, 88km).
The Jubilee Challenge for 300 children with special needs began 30 minutes earlier than planned because of slight concerns about the weather.
Brigadier Steve Hodder said it was quite windy and some of the children were beginning to feel "a bit cold".
The Jubilee Challenge is a seven-mile (11km) route.
All the teams taking part in the Ten Tors spend about six months preparing and need to be completely self-sufficient to complete the challenge.
The event, in its 49th year, is organised by the army, who set up check points at the top of the 10 nominated tors - which are hills topped with granite outcrops.
The challenge was halted two years ago because of severe weather, with teams of youngsters being lifted off the moor by helicopter.
The first time the event was abandoned was in 1996 when Dartmoor was struck by a sleet and snow storm.
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