NO DOUBT, said the historians, the Pension Building once had a fountain, splashing happily away in a pool amid the tall columns that rise like great sequoias to the ceiling 159 feet in the air. But no one had seen the fountain for 40 years. Everyone was absolutely sure, during the various indignities suffered by the grand old building, an unfeeling monster had dug it up and thrown it away.

Cynthia R. Field is director of the Committee for a National Museum of the Building Arts, which mounts exhibits in the Pension Building and plots to take over the entire structure. One day, she was curious enough to pull up a few of the boards on the floor to see if there were any evidence of the old pool. And what do you know? The fountain was right there. In the dim past, a sensitive soul had, no doubt under protest, covered it up, but left all the pieces where they could be found by a future archeologist.

The wonderful brass spray arm, the vocanic block used to surround the works as a sort of sculpture, the red and gray tile floor and the red terracotta lip, and even the volcanic rock, used to make a pyramidial structure to cover the works, were all there, waiting to be discovered. The pool is big -- 28 feet round, 19 inches deep. Of course the flowers in pots which should surround the pool were long gone and no philanthropic flower club or florist's group has yet to offer to replace them.

Field said the other big problem was the plumbing. "We have all the drawings for the building except the plumbing plans." They tried to fill the pool with a hose attached to a faucet from a sink, but "it looked as though it would take two days. We estimated it holds 5,000 gallons."

Somebody suggested calling the fire department. Sure enough, Engine Company 2 sprang to action -- attached a hose to a hydrant outside the building and filled it in a half hour.

The Veterans Administration lent a recirculating pump -- but only for the duration of the current exhibition about Veterans Administration architecture. Field would like to buy the fountain its own pump so she's carefully collecting the money thrown into the pool with wishes. But apparently people don't believe in wishing ponds as much as they used to -- she's only collected $1.37 so far.