MADRID (Reuters) - The discovery of radioactive snails at a site in southeastern Spain where three U.S. hydrogen bombs fell by accident 40 years ago may trigger a new joint U.S.-Spanish clean-up operation, officials said Wednesday. The hydrogen bombs fell near the fishing village of Palomares in 1966 after a mid-air collision between a bomber and a refueling craft, in which seven of 11 crewmen died. Hundreds of tons of soil were removed from the Palomares area and shipped to the United States after high explosive igniters on two bombs detonated on impact, spreading plutonium dust-bearing clouds across nearby fields. Spanish authorities say the appearance of higher than normal levels of radiation in snails and other creatures shows there may be dangerous levels of plutonium and uranium below ground, and a further clean up could be necessary. "We have to study the dirt, we have to look underground," said Juan Antonio Rubio, director general of Spain's energy research agency CIEMAT, which is carrying out an investigation with the U.S. Department of Energy. "We don't know what's down there." The U.S. and Spain have agreed to share the cost of the initial investigation, which is set to begin in November. The governments have yet to agree on who would pay for a clean up, according to a U.S. embassy spokesman in Spain. Spain's government has bought a 10-hectare (25 acre) area near Palomares where the bombs fell. Since 1966, the United States has helped pay for Palomares residents to be checked for signs of radiation poisoning. Spain says there is today no danger from surface radiation. But it still advises local children not to work in fields at the explosion site, nor eat their snails -- which are a local delicacy.