Federal authorities announced one of the largest cocaine seizures in U.S. history yesterday and local Arizona police said they found a pair of tunnels under the U.S.-Mexican border that they suspect were built to sneak drugs into the Southwest. The capture of nearly five tons of cocaine aboard a Houston-bound ship and the discovery of the tunnels in Nogales, Ariz., illustrated two of the ways U.S. land and sea borders continue to come under pressure from international drug traffickers despite interdiction efforts by U.S. law enforcement agencies. The cocaine, totaling about 9,500 pounds, was found last week hidden under a load of iron ore aboard the Cannes, a 580-foot, Greek-owned bulk carrier, after it was boarded by Coast Guard inspectors about 125 miles southwest of Jamaica, officials said. They said the load ranked among the five largest cocaine seizures recorded by U.S. authorities. Its street value was estimated at up to $186 million. In a statement, President Clinton praised the cooperation of the Coast Guard, the Customs Service and the Drug Enforcement Administration in making the seizure and arresting five of the ship's crew. The commissioner of Customs, Raymond W. Kelly, said it is not yet known who owned the drugs, where they were loaded onto the ship or where they were supposed to be distributed. But he cited unspecified indications that crew members might have been planning to remove the cocaine in Mexico so it could then be smuggled overland across the U.S. border in smaller loads. The Cannes, registered in Panama but owned by a Greek shipping company, had taken on 26,000 metric tons of iron ore in Brazil last month and had subsequently stopped in Trinidad on its way to Houston. Most of its 24 crew members were from Ukraine. After the Coast Guard escorted the ship into Houston on Wednesday, Customs agents arrested its Greek captain, two Panamanians and two other ship's officers from Ukraine and Nicaragua, officials said. The rest of the crewmen were being questioned. In Arizona, police in the border town of Nogales found the two suspected drug-smuggling tunnels during a search of a residence and a nearby tool shed. One of the tunnels, 300 feet long with no supports, began in a drainage ditch on the Mexican side of the border, U.S. officials said. Authorities have not yet determined the starting point of the other tunnel, which is about 400 feet long and buttressed with cement and wooden planks. Kelly said the tunnels are in an area of the border known as "a notorious avenue for drug-smuggling." However, police do not yet know whether any drug loads had already passed through them by the time they were discovered. Two other under-the-border tunnels have been found in recent years. One, which led from a private home in Agua Prieta, Mexico, to a lumber warehouse in Douglas, Ariz., was used to smuggle tons of cocaine into the United States until it was shut down after a tip from an informant, DEA officials said. The other, dug 60 feet below ground and stretching half a mile, was intended to come up near the Otay Mesa cargo crossing point in California, but was discovered before it could be used.