As smugglers' tunnels between the U.S. and Mexico grow increasingly common, Washington is pouring resources into the search for a high-tech solution to the problem, when old-fashioned investigative work might be more effective.
On November 16, a security team created exclusively to hunt down tunnels, known as the San Diego Tunnel Task Force, announced the seizure of 17 tons of marijuana after they discovered a passageway connecting the U.S. border city with Tijuana. According to the L.A. Times, the tunnel ran the length of four football fields and descended 20 feet underground. Authoities have discovered over 70 tunnels in the San Diego area since 2008.
The same day, another tunnel was discovered in Nogales, Arizona, bringing the number of tunnels found in that state during the last fiscal year to 12.
U.S. efforts to increase surveillance along the southern border have included proposals to recycle equipment from Iraq and Afghanistan and increase the use of drone aircraft. In a recent Congressional subcomittee hearing by the Department of Homeland Security, several witnesses emphasized one of the U.S. government's most extensive initiatives to improve their monitoring of the border: the use of technology to detect drug tunnels.
U.S. agents have observed increased usage of underground tunnels to smuggle weapons and drugs since the first one was documented in 1990. Since then, authorities have discovered 154 such tunnels, the majority in the San Diego-Tijuana area, although a few have been found in Arizona.
Over time the tunnels have shown increased sophistication, growing in height and length. Several have been discovered equipped with electricity, ventilation and rail systems; others have been used to smuggle migrants.
In response to the problem, the Department of Defense and the Department of Homeland Security have upped their research on ways to detect suspicious activity below ground. Homeland Security representatives recently testified to a congressional subcomittee there are at least four federal task forces dedicated to finding new ways to stop tunnel construction. One project experiments with ground sensors that use seismic waves to detect movement underground, as well as robots that map the terrain using infrared and other technologies.
There a various technologies the U.S. can use to detect tunnels, but all have their limitations. Ground penetrating radar does a poor job at detecting anything before 40 feet. This does little good considering that one tunnel discovered between San Diego and Tijuana traveled at a depth of almost 100 feet below the surface. Ground radar readings are also affected by ground conditions, and give poor results in urban settings or in damp, clay-rich soils.
Other research involves microgravity -- the measurement of minute changes in Earth's gravitational field caused by cavities in the ground. However, the equipment is costly and could give many false alarms. Other technologies using cosmic rays and electrodes have proven to be as equally limited. Tunnels vary greatly in dimension and depth, which also complicates detection through these high-tech methods.
The Department of Defense is reportedly most focused on developing seismic and infrared technology to detect tunnels, although the Department of Homeland Security has observed that such research is slow and "labor intensive." Israel has reportedly developed another method, using fiber optic cables, to track the tunnels excavated in the Gaza Strip and Lebanon. But installing such a system on a mass scale would be expensive and vulnerable to tampering.
The U.S. clearly intends to continue investing resources in tunnel detection research. But it's not clear here that a technological solution is the best one. So far, U.S. authorities have managed to identify tunnels relying on policework and intelligence collection. Resources may be better invested in areas like the cultivation of informants or supporting the investigative work of units like the San Diego Tunnel Task Force.
More research could well be done in developing a seismic detection system, which appears to be the most promising technological approach. But considering that U.S. Border Patrol has defined only 15 percent of the southwest border as strongly secured, it's unlikely the U.S. will develop anything close to control of the underground frontier anytime soon.
See ABC's video report on the latest tunnel discovery, below.