From an underground gymnasium in Washington to a new theater in Fairfax to a new community college in Montgomery County, colleges in the Washington area have been on an unprecedented building spree in the last decade, despite the impending drop in the pool of 18- to 20-year-olds.

In Washington alone, during the decade of the 1970s, colleges and universities will have spent almost a half bilion dollars building educational facilities, probably the largest educational building boom in the city's history.

Currently, Georgetown University is spending $7.4 million on a 3 1/2-acre underground gymnasium that will include a 25-meter pool, a diving tank, 12 multiuse basketball/badminton/tennis courts, an indoor track and a dance studio. A football field with artifical turf surrounded by an outdoor track will be on the roof.

Georgetown also is building a $7.2 million terraced apartment complex for students, sloping down toward the Potomact River from Prospect Street just west of the Georgetown library. "We don't build dormitories any more. We build apartments," says Dean Praice, Georgetown's director of building.

George Washington University is finishing a $200-million building program that has included three new libraries, a classroom building, a student center and an athletic building.

The University of the District of Columbia has plans for a $140-million construction program, including a $70-million downtown campus just north of Mount Vernon Square.

In Northeast Washington, Gallaudet College has $32.8 million worth of buildings now under construction or about to begin, including a new facility for the model secondary school for the deaf, a library, a dormitory and a center for educational technology.

Amercian University will open a new $7.8-million library on January, and Howard University will open a $9-million studen center later this month. A modernistic building overlooking McMilan Reservoir, the student center will include a 12-lane bowling alley, billiard rooms, lounges, a full-service retaurant and an auditorium.

In Northern Virginia, George Mason University, the state university for that region, is about one-quarter of the way into a building program that will cost $100 million to complete.

Currently under construction is a 500-seat theatre for the drama department, an exact replica of the Kreeger Theatre at Washington's Arena Stage, according to building director Joseph Gurfein.

Plans for the George Mason campus near Fairfax City show six major academic areas with about 1 1/2 finished.

Over the next few years, Gurfein says he would like to double student housing. "We opened apartment on campus last year for 500 students and this year we've got a waiting list."

"We need more cafeteria space, classroom space, offices, computer buildings, maintenance buildings and may be even some housing for faculty."

At Northern Virginia Community College, where total enrollment has soared to more than 50,000 from a modest 700 when the school was founded 13 years ago, $7.7 million in new building construction, renovation or addition is under way or about to begin.

With five separate compuses in Alexandria, Annandale, Woodbridge, Manasses and Loudoun, Northern Virginia Community College estimates it served one in every eight adults in Northern Virginia last year.

This month Montgomery College opened its third campus, a $16-million facility on a 200-acre site at German-town near I-270 and Rte. 118.

Designed for 2,000 students, the two classroom buildings have been equipped with solar heating devices that officials say will provide 60 percent of the normal energy needs to heat the buildings and 88 percent of the hot water requirements.

At the University of Maryland in College Park, a new $6.8-million physical education building and a $3.2-million social science building are set to open this fall and a $3.1-million central administration building is to open in February.

Over the next few years, $18.9 million in new construction is scheduled to take place at College Park, including a $5.6-million program of additions and alterations to the mathematics building.