Would-be thieves crashed a bulldozer into London’s Millennium Dome on Nov. 7, hoping to snatch $500 million worth of diamonds-including the spectacular 203-ct. De Beers Millennium Star. The robbers planned to make their getaway in a speedboat waiting on the Thames River. It would have been the world’s largest robbery, but the diamonds were already gone from their tented exhibition arena, and Scotland Yard was waiting. Officers disguised as cleaners overpowered the robbers, and a dozen men were arrested without a shot fired.

Tipped off by police, dome officials had replaced the gems with crystal fakes. De Beers officials say the pear-shaped Millennium Star is arguably the most perfect large diamond in the world.

Four men were arrested after trapping themselves in the vault that usually houses the diamonds. Police picked up another near a speedboat on the river, and a sixth man on the opposite shore. Six others were apprehended later in southeastern England.

Representatives of the New Millennium Experience Co., which operates the dome, said the diamonds are displayed in an exhibition area of the facility called the “money zone” and are closely guarded. The Millennium Star is displayed in a reinforced glass cylinder, and the area has alarms and security cameras. The collection also features 11 large diamonds of a distinctive vivid blue color.

Police said the robbers, apparently disguised as workmen, crashed through a perimeter fence and drove the bulldozer past security guards at around 9:30 a.m., shortly after the attraction opened for the day. As they entered the building, they donned gas masks before unleashing smoke bombs. Fortunately, none of the estimated 100 visitors in the tent were nearby.

Disguised police were positioned around the inside of the dome. After the robbers went into the vaults and became trapped, they were ordered to come out at gunpoint by the officers. The intruders were captured as they bashed at the display cases with hammers and sledgehammers.