By John McPhaul Tue Sep 16, 11:48 PM ET The U.S. coast guard captured a submarine-like vessel equipped with sophisticated navigation equipment and stuffed with seven tonnes of cocaine, Costa Rican authorities said on Tuesday. In a difficult nighttime operation during the weekend, U.S. officials arrested four Colombian smugglers on board the 59-foot (18-meter) steel and fiberglass vessel in international waters before they could sink it. "The boat was partially submerged but you can't call it amateurish. The drug traffickers are not amateurs," Jose Pastor, a spokesman for Costa Rica's public security ministry told Reuters. Several makeshift submarines toting drugs have been captured recently on the high seas. In July, the Mexican special forces captured a similar submarine carrying 200 tightly wrapped packages of cocaine. On Saturday, the U.S. Coast Guard sent a team of special agents on small boats to surprise the smugglers after a U.S. Navy airplane spotted the sub. When the traffickers realized the agents were on their deck they shifted the boat violently in an attempt to throw the officers into the sea. After that failed, they complied with orders not to open hatches designed to sink the craft, said the Coast Guard. "This was the most dangerous operation of my career," Todd Bagetis, the lieutenant in charge of the Coast Guard team, said in a statement. Official photos showed the craft packed full of 37 bales of cocaine with a street value of $187 million. The vessel was likely capable of traveling from South America to the U.S. coast without stopping for fuel or supplies, said the Coast Guard. The U.S. ship that nabbed the traffickers was set to arrive to the Costa Rican port of Caldera on Wednesday with the drug boat in tow, Pastor said.