Narrator: These Marine recruits are training on the rappel ... Rappel tower happens on day 25 of boot camp, a particularly busy day where recruits also train in the dreaded gas chamber.

Narrator: This Marine Corps recruit isn't actually injured ... Narrator: It's part of their final test at boot camp, known as the Crucible, a 54-hour event that all recruits must complete.

Recruit: Aye, aye, sir! Aye, aye, sir! Aye, aye, sir! Aye, aye, sir! Narrator: This is Marine Corps boot camp in Parris Island, South Carolina. Before they become United States Marines ...