The Atlas missile was a highly capable launch vehicle due to its very low structural mass. The place where the greatest mass saving was realized was in the fuel tanks which were, in effect, giant, load-bearing, stainless steel balloons whose strength came not from their dime-thin walls, but from their internal pressure. Without that pressure, the Atlas would have collapsed under its own weight, but, kept properly pressurized, it could throw a circa 1954 hydrogen bomb 5,500 nautical miles, or, more significantly, launch Mercury capsules, and countless other non-lethal payloads into Earth orbit, or beyond (for instance, they launched the first probes to Venus and Mars). But, while it was being developed, the “balloon tank” made a lot of people nervous.
From Rockets, Missiles, and Men in Space by Willy Ley, 1968 edition, pg. 328:
The tank of the Atlas consisted of stainless steel which was no thicker than a dime at any point. Generals—and later congressmen—worrying about what would happen if somebody dropped a wrench on such a tank were conducted to a test version of the “stainless steel balloon” that was stiffened by pressure and offered a choice of assorted mallets to see whether they could dent it. In each case the general or congressman grew tired before he had even succeeded in producing a mark that could be seen.
As I understand it, to this day, if you see an Atlas missile on display, hidden in the background somewhere will be a compressor working (or ready to switch on automatically) 24/7 to maintain the necessary level of pressure in its tanks.