They rappel off an 11-metre (36 foot) height in seconds, run with an open parachute strapped to their shoulders, march with a heavy pack and brave a gas-filled chamber.

It could be a typical day at the South Korean army's Special Warfare Command base in western Seoul -- except that the "soldiers" are teenage boys and young women, often accompanied by their families.

The army's twice-yearly "boot camps" for civilians, offering basic military training for four days, began in 2003 and have proved a big hit.

They are open to anyone aged over 13 who pays 40,000 won ($36) and have drawn more than 17,000 people so far.

Military culture is deeply ingrained in South Korea, ruled by army-backed regimes till the mid-80s. All able-bodied men are still subject to a mandatory two years of conscription to guard against attacks from North Korea.

Boot camps -- run by the military or private firms -- have become increasingly popular in recent years, drawing people ranging from nostalgic veterans to schoolkids, company employees and those seeking special family vacations.

The army says they are an opportunity to "test your limits, enhance physical ability...and learn a strong spirit of 'making the impossible possible'."

"Boys obviously make up the biggest part because they have the mandatory service coming up," said Major Lee Joo-Ho, a boot camp spokesman.

"But more young women are showing an interest, since they were allowed to join a college-based officer commissioning programme last year."

On a typical day this month the muddy training field at Gangseo echoed to squeals, screams and shouts from 230 participants on their third day of training.

In pouring rain, youths practised a mock parachute landing -- jumping into a sandbox with their hands in the air and repeating instructions to land on the balls of the feet.

"Yes, sir!" "I can do it!" they shouted, hitching up baggy, rain-drenched uniform trousers with sneakers peeking out underneath.

"Let me hear your voice! Shout out your girlfriend's name real loud!" a military instructor commands a boy climbing down a cable from a 20-metre platform to simulate a descent from a helicopter.

The nervous-looking trainee murmurs inaudibly, prompting the stern-looking commando to grab the cable and leave him in mid-air.

"Is this what you got? Louder!" he orders repeatedly, until the boy finally yells "I love you, Choi Yoon!" and is allowed to descend.

Fifteen-year-old Yeom Hyuck said he was "very nervous but thrilled" before he hurled himself off an 11-metre parachute jump tower and ziplined to the ground.

"Everything is fun -- but right now I miss my parents," he said.

Kim Tae-Hoon, 17, said his father's dream of joining the army was foiled by poor eyesight. The father pushed his son to attend camp as soon as he turned 13.

Since then, Kim has been back every summer and winter -- a total of nine times.

"This is so good at relieving stress and much more fun than playing computer games," he said.

Since last winter Kim has been joined by his younger brother Tae-Hun, who found it "so thrilling" even to experience the gas-filled chamber, designed to test training against chemical attacks.

"I'm glad I've lost some weight...and I feel more like a man," said the chubby, red-cheeked 13-year-old.

Not everyone was so thrilled.

Cho Byung-Chan, panting hard after rappel training, said he was "a little bit" angry with his parents for sending him.

"They said I need to grow up," said the 15-year-old, who usually spends school breaks playing computer games.

"It's hard...I'm hungry," he said.

Former army commando Yoon Jeong-Sik was spending his summer vacation at the boot camp with his two daughters and wife, 24 years after retiring from the same unit.

Yoon said he wants his family to learn what he did -- self-confidence, pride and how to get along with others.

"So I cajoled my ladies into coming here for character education," said the 47-year-old, water dripping from his hair and soaking his uniform after a mock river crossing.

"At first they were pretty jolly since they had no idea how hard it would be...now I'm trying not to meet their eyes," he said, bursting into laughter with his family.