Hundreds of deported migrants, fearing detection and harassment by police and drug smugglers and unable to return to their homes, have sought refuge in below-ground makeshift dwellings in this Mexican border metropolis.

Around 200 migrants are living in about 30 "pocitos," tunnels up to 15 meters (50 feet) long and a meter deep that have been dug into the rain-softened earth along a section of the Tijuana River near the U.S.-Mexico border.

Some 100,000 people are sent back to Tijuana every year from the United States, making that area of Mexico the recipient of the largest number of deportees.

In that metropolis, a half-mile stretch of the intermittent river known as "El Bordo" -- notable for raised concrete embankments, or levees, on either side of a foul-smelling wastewater conduit -- is a gathering point for some 3,000 Mexican and Central American migrants who became stranded there after being deported from the United States.

Until recently, hundreds of fragile homes erected on the banks of the narrow river were visible at El Bordo, adjoining the San Ysidro district of south San Diego.

But police operations to remove the migrants, commonly by setting fire to their dwellings built out of garbage, plastic and cardboard, began just over a year ago.

Delfino Lopez, a Mexican migrant who looks older than 33, told EFE news agency how one of these operations unfolded: "They told us. The party's over. Now you're going to pay the price.' And we did ... They burned our things. They took everything."

Micaela Saucedo, an activist who directs a migrant shelter, accused the police abusing migrants to kick them out.

"They tried to burn them alive. They doused (houses) with gasoline and set them on fire. Some suffered burns," Saucedo said.

Ismael Martinez, a native of Oaxaca, one of Mexico's most impoverished states, lived for nearly two decades in California. Since being deported, he has not been able to gather enough money to return to his homeland -- more than 1,500 miles away.

When police raids began in El Bordo, it occurred to him that he could dig into the dry river bed and hide away "like a mole" to protect himself from being burned or arrested.

To secure the tunnel and reduce the risk of it collapsing, he began coating the inside of the structure with pieces of wood swept along by the waterway.

Police raids aside, the tunnel dwellers live knowing any day could be their last. -- they could be buried alive by a municipal government excavating machine.

Saucedo recalled that while city authorities were doing clean-up work in the area in late 2012 a digger lifted out the body of a migrant who had been sleeping in one of the pocitos.

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