Two hundred feet below Georgia Avenue at the Beltway, bare rock, mud and dripping water are giving way to the familiar forms of a Metrorail station.

In rude caverns blasted out with explosives, crews are pouring 500 cubic feet of concrete a day to fashion platforms, track beds and service rooms for the Red Line's future Forest Glen station. Set in solid rock, it is the deepest station Metro plans to build.

Work in it and the twin tunnels that lead south toward Silver Spring proceeds by klieg light and in ankle-deep muck. Fresh air pumped down from the surface is quickly scented with diesel fumes belched by a full-size cement mixer truck and earth-moving tractors that rush about in the narrow corridors.

Already, 588 precast concrete panels, each weighing five tons, have been lowered below ground and fitted together to complete the station's vaulting.

Too deep for escalators to be practical, the station will receive its patrons by high-speed elevator instead.

The $77 million job was begun in July 1980. The 290 workers now on the site are scheduled to finish work by August next year. After that, Forest Glen will sit damp and deserted until money becomes available for track, wiring, kiosks, farecard machines and the other dressings of a functioning station. Opening date is the late 1980s, if not later.