An annual sealift by cargo ships as part of Operation Deep Freeze delivers 8 million U.S. gallons (6.6 million imperial gallons/42 million liters) of fuel and 11 million pounds (5 million kg) of supplies and equipment for McMurdo residents. The ships, operated by the U.S. Military Sealift Command, are manned by civilian mariners. Cargo may range from mail, construction materials, trucks, tractors, dry and frozen food, to scientific instruments. U.S. Coast Guard icebreakers break a ship channel through ice-clogged McMurdo Sound in order for supply ships to reach Winter Quarters Bay at McMurdo. Additional supplies and personnel are flown in to nearby Williams Field from Christchurch in New Zealand.