KEY WEST, Florida (AP) — Aboard the Gen. Hoyt S. Vandenberg, a massive World War II ship last used by the US Air Force to track missiles and spacecraft, crews were working hard to sink the ship.

The decommissioned ship is to be sunk today seven miles off Key West to become one of the world's biggest man-made reefs

Explosives attached to the ship's hull beneath the water level will be detonated to open it for flooding, which should quickly send it to the sea floor. The 17,000 ton, 523-foot-long ship will be sunk on a sandy bottom in about 140 feet (43 metres) of clear water.

"Don't go to the bathroom. Don't go get a beer. It should be under three minutes for the ship to fully deploy onto the bottom," said Joe Weatherby, project organiser at Reefmakers, a New Jersey-based company that specialises in acquiring, preparing and sinking craft to create artificial reefs.

It's a project that has been years in the making.

The cost is about $8.6 million, from acquiring the ship to cleaning it. Officials in the Florida Keys expect it to pay dividends, up to $8 million in annual tourism-related revenue, mostly from divers flocking to get a look at the underwater spectacle.

The idea is to not only attract tourists, but to help protect the Keys' natural reefs, already suffering from excessive diving, snorkeling and fishing along with warming ocean temperatures.

Weatherby said people — and fish — will now be drawn to the wreck from nearby natural coral, "giving the reef a breather, which is what it needs."

Preparation for sinking has taken months of inspections and cleanup to remove contaminants. Workers hauled off more than a million feet of wire, 1,500 vent gaskets, dozens of watertight steel doors, 81 bags of asbestos, 193 tons of potentially cancer-causing substances, 46 tons of garbage that could come loose and float to the surface, 300 pounds (136 kilograms) of materials containing mercury and 185 55-gallon (208-liter) drums of paint chips.

The cleanup was performed at two Norfolk, Virginia, shipyards before the boat made the 1,100-mile voyage, arriving in Key West on April 22.

The Vandenberg began as the Gen. Harry Taylor and was later commissioned by the Army as a transport vessel, ferrying troops and supplies from San Francisco to island bases in the western Pacific Ocean in 1944.

In 1945, it carried troops home from Europe near the end of World War II. It was later used by the Navy as a transport ship, and was transferred to the Air Force in 1961, when it was renamed the Vandenberg.

For about 20 more years, the ship served as a missile tracker throughout the height of the Cold War and was retired in 1983.

Organisers say it will serve as "the anchor" to the region's wide array of existing sunken vessels and wrecks from Key Largo to Key West, where some estimate there's a shipwreck about every 300 yards.

The Vandenberg will become the world's second largest intentionally sunk artificial reef. The largest is the USS Oriskany, a decommissioned aircraft carrier nearly three football fields in length that was sunk off Pensacola Beach in northern Florida in 2006.

Mac Monroe, a former mission controller aboard the Vandenberg, said he was pleased the ship won't be turned into scrap metal.

"It's nice to see the old rust bucket again," Monroe said on a recent trip to Key West to see the ship. "And it's a positive outcome for it be sunk and become something useful again."