We got an inside look at what it's like inside the United States Marine Corps' intense 13-week basic training program. Senior video correspondent Graham Flanagan spent five days at the Marine Corps Recruit Training Depot in Parris Island, South Carolina, where he observed different companies at various stages of training. During boot camp, recruits endure a series of intense physical challenges such as entering a gas chamber, rappelling down a 47-foot-tower, and fighting each other with "pugil sticks." All of this occurs under the strict watch of their drill instructors, who create an unrelenting sense of urgency with the intensity and volume of their commands. The Marine Corps is the only US military branch that separates male and female recruits during basic training. In a statement to Business Insider, a spokesperson from the Marine Corps Recruit Training Depot at Parris Island said, "The same gender platoon model allows for appropriate acclimation to the training environment, development of key relationships with drill instructors, and optimal focus during the beginning of their transformation." After weeks of extreme physical and psychological challenges, training culminates with "The Crucible," a 54-hour event where recruits endure exhausting combat scenarios while running on minimal food and sleep.