How to Block a Harbor

Many types of mines have been placed by those who are opposing the Sandinista regime in Nicaragua, some relatively sophisticated and others seemingly handmade. According to U.S. intelligence officials, the 30 to 40 used under the supervision of the CIA have caused the most damage. They are smooth cylinders up to 10 ft. long and 21 in. wide, filled with 300 Ibs. of C-4 plastic explosive. Weighted by another 300 Ibs. of material, they are transported by a mother ship operating 30 miles offshore, carried closer inland by speedboats and dropped to the bottom of shipping channels.

These types of mines have a magnetometer that detects disturbances when heavy steel ships pass overhead. If the disturbance causes the magnetometer to reach a certain threshold, a jolt of electricity from a battery sets off the plastic explosive. After about six weeks, experts believe, the battery loses power and the mines become inactive. Because the mines were placed in deep channels, the explosions are unlikely to cause serious damage to ships. But those familiar with the operation say that "they will crack a seam in the ship, shake things up and knock people around." That is enough to cut off most shipping. "A mine raising spume 50 yds. away," says one official of Nicaragua's Sandinista government, "is enough to make a captain turn around and head to the next port on his list."

The mines being used are far from state of the art. More modern varieties usually have detection and triggering devices that can be programmed to sink only specific types or models of ships.

Officials of the Nicaraguan military say that small unsophisticated mines have been placed in the harbors and in Lake Nicaragua, perhaps by contras operating independently of the CIA. Some are magnetic, others have acoustic triggers, and some merely float near the surface and explode on contact. "These mines are scattered indiscriminately at the entrances of ports," says one Nicaraguan officer. Unlike the large cylindrical mines, these "homemade" devices are not commercially produced. But then" manufacture indicates a relatively high level of technical sophistication. Some are disguised with a rubberized cap that makes them look like rocks, and are set off by the wake of a passing ship. Says Eden Pastora, commandant of the wing of anti-Sandinista rebels that claims responsibility for setting explosives in Lake Nicaragua: "We made all the mines ourselves with simple materials that can be purchased on the worldwide black market." A Nicaraguan military official, however, says that most of the activity is directed by the U.S.

His view: "The mining requires special skills that only the gringos have."