The U.S. Navy found that Iraq had made elaborate plans to lay naval mines outside it's only port, Umm Qasr. Several small patrol boats were captured when Umm Qasr was quickly overrun early in the war. In the rear of these small boats were found hidden naval mines, that could be quickly put overboard into waters off the port. A barge was also found, that had been modified internally to hold 90 mines, and to put them into the water without being seen. Two dhows (wooden sailing vessels) were found, each modified to carry six naval mines. Most of the mines were "bottom mines," that lie on the sea bed in shallow water and explode when they detect the pressure from a ship passing overhead. Iraq bought many Italian "Manta" bottom mines for about $10,000 each in the 1980s. Two of these mines damaged two U.S. warships in 1991. Iraq has since produced it's own copies of the Mantas, called "Sumer" mines. Bottom mines are very difficult to detect, and to help with ensuring that none were still around Umm Qasr, the U.S. Navy flew in two of their mine hunting dolphins. The only problem they expect to encounter with the dolphins is hostility from local dolphins. Like wolves and bears, dolphins are territorial and will try and chase away any strange dolphin they find in their normal feeding territory.