Last week when rangers locked the doors to the cavernous underground vaults beneath the Lincoln Memorial, where they give nightly flashlight tours, they returned to find the doors open. When they turned off lights, they found them on just moments later. Or when they turned them on, they saw them go off.

National Park Service rangers may not believe that ghosts, gremlins, goblins or poltergeists inhabit the 60-year-old marble monument, but nonetheless the locks on all the doors were changed Thursday, the damp underground grottoes have been thoroughly searched and armed U.S. park policemen have accompanied the flashlight tours for the last three nights.

"It's unsettling, you just want an explanation," says Ranger Bonnie Blaford, a native of Illinois who has worked at four Lincoln historic sites and never before encountered any "unusual occurrences like these.

"We've had trouble all summer with lights," Blaford says. "After all, they were installed in 1928. But in the past week I have physically turned off a light underneath, come back and it was on. And we have found doors open" that have just been shut.

The search was made to see if someone was living under the memorial, but no human evidence was found except for the discarded shoes, bottles and litter left by workmen prior to completion of the memorial in 1922.

Now, when Blaford and other rangers lead the flashlight tours, the only underground tours for the public in Washington, they carry walkie-talkies and go in pairs -- in addition to having the U.S. Park Police officer tag along on the tours.

"Why is that policeman following us?" asked a voice in the dark on Thursday night's tour, which members of the architectural firm of Keyes, Condon and Florance had reserved. It was one of the few questions that was not answered by ranger Connie Shemro, who led the tour among the stalactites and stalagmites beneath the memorial steps and among the huge cartoon-festooned columns that keep the memorial itself from sinking in the mud.

The icicle-like stalactites and stalagmites -- called "soda straws" by the rangers -- have been formed from dripping water full of lime, leeched over the years from the concrete in the steps and plaza at the memorial, the city's most popular monument.

The caricatures of Mutt and Jeff, President William Howard Taft, memorial architect Henry Bacon and assorted people were apparently inscribed by workmen during the eight years it took to complete the memorial and sink pilings down through marshy West Potomac Park to bedrock. The drawings were noticed in 1975 by Park Service officials as they installed an elevator for the handicapped. The tours beneath the memorial, begun in 1977, are limited to groups of 20 and reserved months in advance.

The Park Service has a dozen rangers, 10 of them women, giving tours seven days a week at the Washington Monument and Jefferson and Lincoln Memorials. Some rangers, like Cathy Bowman, are reticent in discussing the paranormal physical phenomena, as such strange happenings might be called.

"In any old building things happen. There are unusual occurrences but it's no big deal. I'm not going to give you anything," the tight-lipped Bowman told a reporter.

Bowman's boss, Robert Miller, assistant director of Mall operations, has done some psychical research into the Lincoln Memorial's "unusual occurrences" and concluded that "some things can be explained by human error such as doors left unlocked and some we can't explain such as the lights going on and off ."

Nonetheless, to reassure his rangers about the "funny things" occurring at the Lincoln Memorial, he had the locks changed on three doors that lead to the subterranean vaults and got the Park Police officers to go along on the hour-long tours.