ROME (AP) — The cruise ship grounded off Tuscany shifted again on its rocky perch today, forcing the suspension of diving search operations for the 21 people still missing and raising concerns about the stability of the ship’s resting place.

It was not clear if the slight movements registered by sensors placed on board the Costa Concordia were just vibrations as the ship settles on the rocks off the Tuscan island of Giglio or if the massive ocean liner is slowly slipping off the reef.

The sensors detected that the ship’s bow was moving about 15 milimeters an hour and the stern about 7 millimeters an hour, said Nicola Casagli of the University of Florence, who has been called in by Italian authorities to monitor the ship’s stability.

The Concordia’s movements are being watched since any significant shift could be dangerous for divers trying to locate those still missing after the Concordia ran aground Jan. 13. An additional fear is that movement could damage tanks holding a half-million gallons of fuel oil and lead to leaks.

The sea floor drops off sharply a few yards from where the ship is resting, and Italy’s environment minister has warned it risks sinking altogether. Storms forecast for later today have added to concerns.