Nine-year-old Sebastian's mom makes the sign of the cross over him before sending him to school, reminding him not to stray from the path through the mine-strewn field along the way.

Sebastian lives in an area where the Revolutionary Armed Forces of Colombia (FARC), the Marxist guerrilla group that has waged a half-century war on the country's government, used to have a base.

Today, the abandoned guerrilla camp in Cocorna, in the northwestern department of Antioquia, has been turned into a school.

To get there, Sebastian has to walk half a kilometer (500 yards) through a field sown with anti-personnel mines, which have claimed more victims in Colombia than in any other country in the world outside Afghanistan: 2,000 killed and 9,000 wounded since 1990, 10 percent of them children.

"I warn him before he goes out not to set foot anywhere off the path," Sebastian's father, farmer Argiro Duque, told AFP, saying he is "very afraid" of the buried mines.

An army battalion is currently at work sweeping the village for mines.

But until they finish, the children are confined to small patches of safe space.

Sebastian, who loves football, plays often with his friends on the playground at school. But they are strictly forbidden to step off the pitch, into the territory beyond the fence where red signs with skulls on them warn the area is mined.

"We're always losing the ball. We ask the soldiers to please get it for us. Sometimes it sits there for two or three days," he said.

If he tries to get it himself, he said, he knows he could end up "legless, headless, lifeless."

His teacher, Edwin Ramirez, tried to shield them from the truth, telling them they had to stay close because there were wild animals in the trees.

He regrets that "the playground is at once theirs and not theirs," he said with a shake of his head.

But despite his efforts to protect their innocence, most of the students know the real reason for their confinement.

- Invisible threat -

Nearly all the actors in Colombia's messy conflict have used mines at some point, from guerrilla groups to right-wing paramilitaries to the army.

The military stopped using them after the government signed the Ottawa Treaty against land mines in 1997. It has accused the guerrillas of continuing to use them.

Now, for the first time, the army and the FARC have begun working together to clear land mines as part of peace talks they opened in November 2012.

They started in the valleys of Antioquia, a region known for its coffee plantations and flower farms.

Of Colombia's 1,120 municipal districts, more than 60 percent are believed to be mined, sometimes in highly trafficked areas.

The international organization Halo Trust, which is taking part in the mine-clearing effort around Cocorna, found two mines buried near a statue of the Virgin Mary in one of the only spots in town that has a decent cell phone signal.

The FARC's mines are often improvised -- coffee tins filled with shrapnel, for example.

In the nearby village of Sonson, a former guerrilla has begun clearing mines with a metal detector. He was 15 when FARC fighters killed his father in front of him and forced him and his sister to join them.

After escaping from the rebels, he says he is now "proud to know that people will be able to set foot on a patch I cleared and nothing will happen to them."

His neighbor Marta is supervising Halo Trust's work at a school that was abandoned after right-wing paramilitaries killed a teacher there.

She recounted how, as a child, she stepped on a mine that failed to explode.

Her 13-year-old neighbor wasn't as lucky.

"He went to count the cows and stepped on a mine. They found him dead, lying in his own blood," she said.

Clearing the entire country will be a long, difficult job, she warned.

"No one knows where the mines are. And unfortunately, we villagers are the hardest hit, even though we have nothing to do with this war," she said.