Tropical Storm Isabel wrought tens of millions of dollars in damage at the Naval Academy, destroying lab equipment, plumbing and ventilation systems, and leaving half the classrooms unusable, school officials said yesterday.

Marines and civilian workers had sandbagged buildings last week, and some faculty and staff had moved computers and other valuables off the ground floors. But academy officials said yesterday that they had underestimated the force of a storm that at its peak left parts of the campus 8 feet underwater.

"We are still in disaster-recovery mode," Cmdr. Rod Gibbons, the academy spokesman, said last night. "The extent of flooding damage is unprecedented."

The 338-acre Annapolis campus lies where the Severn River flows into the Chesapeake Bay. Significant parts are built on landfill that is particularly vulnerable to flooding.

Academy officials were still assessing the damage, so no dollar estimate was available.

Nearly half of the 4,000-student dormitory and several athletic buildings remained without hot water yesterday. Several academic buildings were so water-logged that it was unsafe to restore power.

Gibbons said the military college expects to have hot water and electricity back within a few days. But it may take months to repair the central air conditioning plant and weeks to restore heat.

The basements and ground floors of Rickover and Chauvenet halls, the school's engineering and science buildings, suffered particularly severe damage. Floodwaters ravaged the chemistry lab.

Professors have moved their classes to gyms, libraries, auditoriums and other dry spots.

More than 100 contractors have been working around the clock, pumping out flooded basements, repairing electrical lines, disinfecting buildings and ripping up drenched carpets.

The swimming pools at Lejeune Hall were off limits, because storm water was still ceiling-deep in the below-ground machine rooms that pump and filter the pools.

Despite damage to grounds and buildings, academy officials said they were successful in protecting students and boats.

Midshipmen were told to stay inside Bancroft Hall during the storm. They emerged from the dormitory Friday to help clear fallen branches and limbs. No one at the academy was injured.

The school's 108-foot Yard Patrol boats were moved to hurricane moorings in the upper Severn River last week and escaped harm, officials said.

"This is the worst we expect it to be," Gibbons said. "Every day, we expect to get better."