Earlier this month, the Coast Guard Cutter Valiant intercepted a 40-foot long semi-submersible vessel carrying 12,000 pounds of cocaine and four smugglers. Vessels of that sort, commonly referred to as “narco subs,” travel while almost completely submerged in order to limit detection from the sea. As a result, the Coast Guard and American law enforcement rely on maritime patrol aircraft to spot these subs from above.

“There are no words to describe the feeling Valiant crew is experiencing right now,” said Cmdr. Matthew Waldron, Valiant’s Commanding Officer. “In a 24-hour period, the crew both crossed the equator and intercepted a drug-laden self-propelled semi-submersible vessel. Each in and of themselves is momentous events in any cutterman’s career. Taken together, however, it is truly remarkably unprecedented.”

According to a Coast Guard release, the Valiant was redirected to intercept the 40-foot submarine after it was spotted by a nearby aircraft. The order, which came from the Joint Interagency Task Force South (JIATF-S), came just before sunset on an as-yet unspecified date in September. The Valiant deployed two small boats, each with its own team comprised of crew from the cutter itself as well as personnel from the Coast Guard’s Tactical Law Enforcement Team. Shortly after they began the operation, the Coast Guard vessel was joined by “Colombian Naval assets,” though no further elaboration was provided regarding any support the Colombian Navy may have provided.

“Approximately over 1,100 pounds of cocaine were recovered and offloaded to the Valiant during the operations. The remaining cocaine on the semi-submersible could not be safely extracted due to stability concerns of the vessel,” the release read.

“Narco subs” have been a common facet of the drug smuggling enterprise from South America for years, but the vessels continue to become more advanced. These semi-submersible ships are often made almost entirely out of fiberglass, with the engine accounting for most of the cost of construction (which is estimated to be around $1 million per vessel). A million dollars may seem like a lot to spend on a dangerously unstable and unsafe method of transportation, but with $12 million worth of drugs on board, delivering the goods would easily cover the investment.

In order to avoid detection, some newer narco subs have even been found with lead lining around the interior of the vessel, hoping to reduce the infrared signature produced by the engines and occupants.

“We’re seeing more of these low-profile vessels — 40-plus feet long … it rides on the surface, multiple outboard engines, moves 18, 22 knots … and they can carry large loads of contraband,” Coast Guard commandant Adm. Karl Schultz told Business Insider in an October 2018 interview.