Three score and three years ago our city planners brought forth on the former marshes of the Potomac River a monument, dedicated to the proposition that Abraham Lincoln should have a memorial at the end of the Mall and conceived in the hope that it would'nt sink into the mud and its visitors wouldn't be eaten alive by mosquitoes.

To keep the memorial from sinking, a huge hidden cavern of cement and steel columns was constructed, like flying buttresses in a midieval cathedral, on top of an even deeper underground structure of 122 cement columns reaching another 60 feet down to bedrock.

It is here in the damp, Stygian darkness, where stalactites now are nine feet long and the World War I cartoon drawings of construction workers are still scrawled on the walls, that the "other" tour of the Lincoln Memorial begins.

"It isn't anything fantastic like the Lurray Caverns," National Park Service Ranger Jim Burgess warned a group of 15 flashlightoting visitors earlier this month on the first of the new nighttime underground tours. "But it's a side of the Lincoln Memorial you don't often see." It is also the only underground tour in the Nation's Capital, certainly the only one with stalactites and stalagmites.

A partial glimpse of what holds up the 175-ton statue of Lincoln and the solid marble temple that surrounds it was afforded tourists last year when the Park Service built an elevator for the handicapped inside the base of the memorial. The park official in charge of Bicentennial events, Leroy Rowell, suggested putting a window in the wall to show tourists the huge underground cavern.

The view apparently whetted the appetites of spelunkers and those who like off-beat tours. A few trial underground runs last April convinced the Park Service to make it a regularly scheduled event. The tours are now held twice weekly, on Wednesday and Thursday nights, but reservations are required since only 15 persons - in old clothes and good shape - can go on a tour.

It is not a trip for the aged or handicapped since about 50 three-foot, cement-encased steel beams must be clambered over in dark, dank, 80-degree heat in order to see stalactites and stalagmites beneath the plaza facing the Reflecting Pool. The thousands of icicle-like stalactites, formed by water dripping through the monument's marble steps, are called "soda straws" because they are hollow and about the thickness of a straw even when 9 feet long or longer.

"If you want to get deeper into caving, you can one of the grottoes of the National Speleological Society." Ranger Burgess told his perspiring group.

Besides the soda straws and their companion stalagmites on the ground, which look like fried eggs at first and after 50 years like castles in the sand, the highlights of the underground tour are the huge columns themselves and the grafittl which adorns them.

"We have here what appears to be some doodles by an artistic but unidentified construction worker. This is either Mutt and Jeff or William Howard Taft and Henry Bacon, the chairman of the memorial commission and the architect, probably done between 1914 and 1922 during the construction period," said Ranger Burgess.

"This lovely drawing in top hat and cigarette holder is of Woodrow Wilson. President from 1913 to 1921, and this one, which you'll see in a number of places, we think is the construction foreman."

"Are there any bats down here," a voice asked in the dark behind Ranger Burgess? "There are bats upstairs taround the monument but not down here. However, there are spiders, moths and other bugs that live in the total darkness," Burgess said, flashing his lights on a small web overhead.

"The spider web was what fascinated me," said Irmgard Svenson outside the memorial after the hour-long trip was over. "We have been here eight years and want to see everything we can of Washington, up and down," said her husband. Erik. "I've come away with a new respect for the construction of the memorial."

Jim Dougherty, a young Capitol Hill attorney who heard the tour being touted on a 3 a.m. jazz program and pedaled down for the night tour on his bicycle, said "it was fascinating, like a dark closet."

the tours have been booked for the rest of October but November dates are still available, according to Ranger Burgess. Reservation can be made by calling the Park Service's Mall operations headquarters at 426-6841.