An interior view of the Apollo 13 Lunar Module and the "mailbox." The "mailbox" was a jerry-rigged arrangement which the Apollo 13 astronauts built to use the Command Module lithium hydroxide canisters to purge carbon dioxide from the Lunar Module. Lithium hydroxide is used to scrub CO2 from the spacecraft atmosphere. Since there was a limited amount of lithium hydroxide in the Lunar Module, this arrangement was rigged up using the canisters from the Command Module. The "mailbox" was designed and tested on the ground at the Manned Spacecraft Center before it was suggested to the problem-plagued Apollo 13 crewmen. Because of the explosion of an oxygen tank in the Service Module, the three astronauts had to use the Lunar Module as a "lifeboat."