The annual two-day Ten Tors walk across Dartmoor has been abandoned half-way through because of heavy rain.
The Army began escorting hundreds of teenagers off the moor at first light, with 112 of the six-person teams evacuated by 0813 BST.
All the 2,400 participants are expected to have been taken off the moor by mid-afternoon on Sunday.
Among those involved were team-mates of a teenage girl who died training for the event, now in its 47th year.
Charlotte Shaw, 14, from Edgehill College, north Devon, died in March after falling into a swollen stream.
The 400 teams of six young people on the moor range in age from 14 to 20 and compete on 35, 45 and 55-mile routes.
Two Sea King helicopters from RNAS Yeovilton in Somerset joined the operation on Sunday morning, taking four teams in each airlift.
Others were walking off the moor or being taken off by road to Okehampton Camp in Devon. Parents were asked not to pick up their children until midday.
One participant, Rosie, told BBC News: "We have been training for this for so long.
Helicopters from RNAS Yeovilton joined the airlift
"We are so disappointed."
A statement from the organisers said the decision to end the event early was taken at 2130 BST on Saturday because of "deteriorating weather conditions".
The decision was also influenced by the high drop-out rate of 15% on Saturday.
An Army spokesman said: "Our priority is to ensure all the people involved are as safe as possible. The weather forecast indicates that the rivers, which are already at their peak, will become increasingly swollen."
"All participants will remain where they are currently camped - safe and with shelter and food.
"They will be moved off the Moor in an orderly and controlled manner."
The event was last severely affected by bad weather in 1996 when 2,100 of the 2,400 taking part were evacuated.
Three boys and eight girls from Edgehill, the college attended by Charlotte Shaw, were in this year's event.
Charlotte died when she was swept away after she dropped some of her kit into the Walla Brook. She fell in when she leaned over to retrieve it.
Edgehill College head teacher Stuart Nicholson said they had been given special permission by the organisers to enter a team larger than the normal six members.
Speaking before the event, he said: "We have worked very closely with all concerned to understand the emotional impact both of participation and of not participating.
"As a school, we have made it possible for the group to continue the usual training, with additional support, whilst recognising that at any stage any of them might feel unable to continue.
"In practice, as the weeks have gone by the group has become more determined that they want to take part.
"We know that Charlotte's spirit of endeavour will be with them all."
For the first time each team was allowed to carry one mobile phone, for emergencies only.