The Saturn V was huge, and huge rockets tend to have proportionately devastating explosions. Engineers calculated that a Saturn V exploding on the launch pad would turn into a fireball 1,408 feet (430 meters) wide and burn for nearly 40 seconds reaching a peak temperature of 2,500 degrees Fahrenheit (1,380 degrees Celsius).

In the age of Saturn V — the 60s and 70s — to get astronauts and launch crews clear of a fatal explosion, NASA had three possible escape plans in place: the launch escape system that would pull the command module free from the rocket during an abort; a slide wire astronauts could ride to a safe point on the ground; and an underground blast chamber.

ANALYSIS: Apollo 11′s Unconventional Life Insurance Policies

The blast chamber is somehow buried in all the Apollo-era history. It's fitting, perhaps, since it's actually directly beneath the launch pad where the theoretical Saturn V explosion would have occurred.

The blast room is basically a bomb shelter. A small, circular room, it's mounted on massive springs like a missile silo. This meant that anyone inside would have felt little disturbance when the Saturn V exploded right overhead.

Lining the room are huge chairs, big enough for an astronaut in a full pressure suit to strap himself in for safety. There's also one fire blanket per man in the center of the room (shown below).

Up to 20 men could seek refuge in the blast chamber for up to 24 hours, though with more men, things became problematic due to the rise in carbon dioxide levels. The room was equipped with carbon dioxide scrubbers that came with spare filters and a store of oxygen candles — a type of chemical oxygen generator containing a mix of sodium chlorate and iron powder that burns to produce 6.5 man-hours of oxygen per kilogram of the gas mixture.

ANALYSIS: Apollo 12′s Lightning Launch Chaos

At the time, on the wall was a detailed schedule outlining exactly when oxygen candles had to be lit and filters had to be changed. With less than six men in the blast room, they could all breathe normally for a full day while the air above them cleared. With up to 10 men in the room, things got a little more complicated. Additional methods of providing oxygen became imperative if everyone inside was going to survive.

As evidence that men could last for a while in the blast room, there was even a toilet. But barely tucked away behind one of the chairs, using it in such a small space wouldn't have been an appealing prospect.

To get into this fortress of safety, the astronauts and pad crews had to take a ride. Elevators would carry them from any level on the gantry to the base of the mobile launch platform where, on the north side, was a square door with rounded edges. It opened to a slide, 200 feet (60 meters) long, that would send astronauts and pad crews on a winding ride to a point 40 feet (12 meters) under the launch pad. They landed in the rubber room, so called because it was padded entirely with bouncy rubber. A six inch steel door admitted them through a short tunnel and into the blast chamber.

Once the air around the launch pad had cleared and it was safe to leave, astronauts and pad crews could take one of two long, narrow, and winding tunnels to the western edge of the launch pad area. There, they could open a door and step outside.

ANALYSIS: Nixon's Contingency Plan for a Failed Apollo 11

After the Apollo program ended, the rubber rooms and blast chambers were abandoned in place. There were no circumstances under which shuttle astronauts would use this underground shelter; the preferred method beginning in the 1980s were the gondolas on cables that led from the top of the gantry to a safe site on the ground.

The rubber room and blast chamber, at least the one under Pad A, is still there. It's off limits to the public and preserved as a historic site, but if you can finagle your way in (which involves knowing the right people) it's definitely a piece of history worth seeing.

To see NASA's rubber room in all its glory, watch a clip from a BBC documentary on the Apollo emergency escape procedure.

Image: Top: The door to the rubber room. Middle: The rubber room with fire blankets in the center. Credit: NASA