One of the most feared terrorist attacks is on a liquefied natural gas tanker (LNG) delivering it's cargo to the three ports in the United States that handle it (Boston, Savannah, Georgia and Lake Charles, Louisiana.) The gas is cooled to 260 degrees Fahrenheit, so that it becomes a liquid and occupied a lot less space. The ships that carry LNG are basically floating refrigerators. Getting one of these to explode would be very difficult. There has never been a LNG fire aboard an LNG tanker. The last time there was an LNG accident (in 1944) it was at a storage facility on land, and the liquid gas had escaped and burned, rather than exploded. In theory, getting an LNG tanker to "explode" would produce an effect similar to one of the atomic bombs dropped on Japan during World War II (at least 20 kilotons.) But it is more likely that a breach in an LNG tanker would lead to a fire, not an explosion. But the U.S. Coast Guard is taking no changes. For example, each LNG tanker entering Boston harbor is escorted by six Coast Guard gunships and three helicopters.