Smuggling a human being from Mexico into the US is relatively simple—only 670 miles of fencing protect the 2,000-mile border. But smuggling a truckload of drugs? That’s a bit trickier. To bypass checkpoints, dope haulers have been taking their underground trade even more underground, digging passageways to connect the two countries. Last November, two tunnels from Tijuana, Mexico, to San Diego were discovered—along with 40 tons of marijuana. And in the past two years, authorities in the Arizona border town of Nogales have found 27 illicit conduits joined to the city’s storm drains. Turns out spotting them is hard: Urban noise and clay-rich soil mask the tunnels from radar.

So to sniff out the tunnels, law enforcement has decided to dig a few of its own. With funding from the Department of Homeland Security, R&D firm Mitre is working on a system that combines radar traditionally used to find oil and gas deposits with the telecom industry’s methods for laying fiber-optic cable. First, they dig horizontal boreholes. Then omnidirectional radar is shuttled up and down the channel, where there’s little interference from surface sound, making it easier to detect tunnels. What does the shuttling? “A robot,” says Weiqun Shi, Mitre’s principal investigator on the project. “It pushes and pulls antennas through the borehole really, really fast, like a few miles per hour.” Mitre is gearing up for field tests. Soon drug moles may be neutralized by robotic tunnel rats.