The theft from the Museum of Natural History in Mdina was discovered Tuesday during a routine check, officials said. A protective cover of plastic had been forced open to take the rock, which was the size of a raisin.

The rock was picked up in a lunar valley named Taurus-Littrow during the Apollo 17 mission in 1972, the last of the Apollo moon-landing missions. It was one of many moon samples given to nations of the world by the United States.

The exact value of the rock wasn't known but a similarly sized moon rock in Honduras, from the same Apollo mission, is worth about $5 million. That rock was stolen sometime between 1990 and 1994 and was recovered in 1998 after a sting operation.

A Maltese flag displayed next to the rock -- which the U.S. astronauts had taken up with them -- was not taken.

"The problem the thieves have is what to do with it," Joseph Richard Gutheinz, a retired NASA agent who helped recover the Honduras rock, wrote in an e-mail to The Associated Press. "They can try to sell it to private collectors or if they're sufficiently dumb, at an auction house."

There are no surveillance cameras and no custodians at the Museum of Natural History because of insufficient funding. The only attendant is the ticket-seller.