Do bunkers have to support the weight of earth above them in order to avoid collapse?

200 feet below the earths surface lies a man-built cavity, a bunker. the cavity is 10 feet tall, 40 feet wide, and 50 feet long... for a total floor area of 2000'... with it being 2000' in top surface area and 200' below the ground there is 400,000 cubic feet of dirt on top of this cavity, each cubic foot weighs approximately 45 pounds, for a total of 18,000,000 pounds of dirt on top of this cavity... does this cavity have to bear 18 million pounds on its "roof"? if so, how do engineers combat this? what are bunkers built this deep (ive heard bunkers can be built up to 1 km below the surface) made of? not to mention they also have to be built to withstand artillery fire/bunker-busting missiles.