Standing under one of the high vaulted ceilings of Fort Knox in Prospect, Ranger Michael Leighton reached down to the damp stone floor and scooped up a few small pieces of crumbled brick.

“This is how the fort is falling apart,” he said. “We just sweep it daily as it falls from the ceilings.”

Fort Knox, which never was fired upon in its 140-year history at the mouth of the Penobscot River, is slowly being destroyed instead by water damage from a leaky roof.

The deterioration of Maine’s largest historic site is not evident to most of the 70,000 visitors who each year explore its dark passageways and magnificent spiral stairways. Built of brick and massive granite blocks, the fortress appears to be as solid now as it must have appeared to the soldiers who occupied it during the Civil War and the Spanish-American War.

But to caretakers such as Leighton, who has managed the Fort Knox State Park for the last eight years, the steady decay is sadly apparent. Lime that has leached out from the mortar now hangs from the ceilings like stalactites, oozes from the corners of brick like a hard white foam, and stains every wall throughout the pentagon-shaped fort.

“It’s very depressing,” Leighton said. “You see a hairline crack one year and it’s an inch-wide the next, and you have to ask how much longer before we have to close down parts of the fort.”

Maine voters will be asked in November to approve a $5 million bond request by the Bureau of Parks and Recreation for public-safety improvements in its park system. Of the total, $1.5 million would be used to fix the faulty roof of Fort Knox, the first in a long list of improvements that the department said is critical to the future of the national historic landmark.

On Aug. 25 and 26, Company B of the 20th Maine Voluntary Infantry Regiment re-enactment group, will camp at the fort with other Civil War re-enacters from throughout New England to bring attention to the needs of the fort.

“It’s one of the most heavily visited state historic sites, if not the most, and it’s important for us as stewards of the fort to do whatever we can to preserve it,” said Sheila McDonald, a resource administrator with the parks department. “It’s a massive job and a little overwhelming, but we think we have the solution to deal with the problems.”

Those problems are nearly as old as the fort itself, which was built to protect the wealthy upriver lumber town of Bangor from British warships during the “Aroostook War.” Construction began in 1844 and ended nearly 25 years later, at a cost of $1 million.

According to old military reports, the army began complaining about the leaking roof as early as the mid-1880s.

The original roof had been designed to let rainwater seep down through the sod covering, flow over the brick arches and into drains, and be carried into cisterns below. But the pitch coating used to waterproof the arches deteriorated quickly, allowing water to enter the brick and leach out the lime that was used as a binder in the mortar.

As the weakened mortar crumbled over the years, deep gaps have formed between many of the bricks in the vaulted ceilings. The iron rods that anchored the fort’s granite shell to the inside walls have also deteriorated, and many wood timbers have rotted.

“Two areas of the fort were closed this month because of public-safety concerns,” Leighton said. “And I have been asked to think hard about other places that might have to be closed. It’s a shame.”

Fort Knox was last occupied in 1898, during the Spanish-American War, and purchased by the state in 1923, along with five other abandoned forts. Its guns were bought by towns and cities all over Maine, and others were hauled off for scrap.

More than 120 acres of land were groomed as a state park and the fort was opened for public visitation. After an unsuccessful effort to repair the original roof in the 1950s, workers tore up the old sod 10 years later and poured an asphalt roof to shed the water. The asphalt cracked as it expanded and contracted, however, and the leaking continued.

Government and private engineers, including the Canadian engineer hired to oversee the $40 million restoration of the Citadel in Halifax, concluded that Fort Knox would distintegrate in stages if the leaking persisted.

Leighton said the state now plans to remove the asphalt roof, resurface the arches with modern waterproofing materials, and install a turf covering similar to the original. Concrete slabs will be used to secure the fort’s outer and inner walls, and timbers will be replaced. Once the roof is fixed, the state hopes to begin the massive job of re-mortaring the damaged bricks that form the inner shell of the structure.

“It’s a long-term project, no doubt,” McDonald said. “but the biggest component is the roof. After that, the first major leap will be made into preserving one of our premiere historic sites.”