Smugglers continue to construct tunnels beneath U.S. borders to transport drugs, illegal aliens and other contraband, according to an internal briefing prepared by a U.S. Northern Command Task Force.

Dozens of tunnels have been found in recent years, including some of remarkable sophistication, but it is likely that others remain undetected. Overall, between 1990 and November 2008, 93 cross-border tunnels were discovered, a Task Force briefing slide stated (pdf). Thirty-five of those were in California, fifty-seven in Arizona, and one in Washington State.

Some of the tunnels are primitive, “hand dug” affairs. Others are the product of surprisingly ambitious and complex engineering projects. In one extraordinary case in 2006 (pdf), a tunnel was discovered near Otay Mesa in California that began with a 90-foot deep vertical shaft on the Mexican side that gradually ascended to an exit point in California more than half a mile north. Seven feet in height, electrical power and ventilation were provided throughout the tunnel. “This tunnel was the longest yet found under the U.S. border,” the new briefing indicated.

The Task Force briefing, which has not been approved for public release, was inadvertently posted on the Internet by U.S. Northern Command before being withdrawn last week. A summary slide from the briefing on “Tunnels Since 1990” is here.

A descriptive tabulation of tunnels that were discovered between 2005 and early 2008 (based on data from the Department of Homeland Security) is available here.

At least six new tunnels were discovered in the first quarter of FY 2009, the Department of Homeland Security reported last December.

“The proliferation of tunnels dug underneath the border” may be “another unintended consequence of the border fencing” that has been erected along portions of the U.S.-Mexico border since 1990, a Congressional Research Service report (pdf) suggested last year.

Last week InsideDefense.com reported that the same Northern Command Task Force briefing was critical of Canadian immigration policies, which it said were too hospitable to potential terrorists. See “DOD Officials See Terrorist Threat to America Brewing in Canada” by Sebastian Sprenger, March 20.