Basic Underwater Demolition/SEAL (BUD/S) training is a six-month selection process and the gateway into the Navy’s SEAL Teams.

Broken into three phases (First Phase, Second Phase, Third Phase), BUD/S has an attrition rate of between 70 and 85 percent. The complete SEAL pipeline attrition rate — from the moment someone walks into the recruiter’s office to the end of advanced qualification training — is over 90 percent.

First Phase is the basic conditioning part of BUD/S. Students learn to work as a team under increasingly difficult physical and mental conditions, with four-mile timed runs, obstacle course timed runs, and two-mile timed swims determining who stays and who goes. This phase is where most students’ dream of becoming a SEAL ends. This is also where Hell Week takes place.

Second Phase is the dive part of BUD/S, where students are introduced to basic combat diver operations. The pool competence test in this phase is arguably the second-hardest event during BUD/S, after Hell Week.

Third Phase is the land warfare part of BUD/S. Students receive basic training in marksmanship, explosives, land navigation, and small-unit tactics. This phase mostly takes place on California’s San Clemente Island.

Navy SEAL Hell Week

Hell Week takes place during the fourth week of First Phase. (The exact time in the training has shifted several times during BUD/S’s history.)

Hell Week lasts almost six days — Sunday evening to Friday morning — during which students run more than 200 miles, often with boats on their heads, swim, do hours of physical training with logs, and numerous other brutal evolutions. They are constantly wet, cold, and sandy and only get about four hours of sleep throughout the week.

During Hell Week, students are fed regularly and consume over 8,000 calories a day but still lose weight.

Some of Hell Week’s events have been the same since its inception back in the days of the Underwater Demolition Teams (UDTs) of World War II.

“My class started with 150 students, and when we got to Hell Week, 50-something remained,” a former SEAL officer told Insider. “At the end of Hell Week, only 24 were there, some of them pretty beat up.”

Breakout!

On Sunday evening, the students move from their barracks to tents, bracing for the coming ordeal. Suddenly, thousands of machine-gun blanks and artillery simulators go off as “wolf packs” of instructors seek to shock and awe the students.

The next minutes involve hundreds of push-ups, sit-ups, flutter kicks, all while being wet and sandy. There are regular but controlled visits to the soul-chilling Pacific for “surf torture,” where students lock arms and lie on the surf for minutes at a time. Hell Week or not, the cold claims a lot of students.

As the week progresses, the instructors put the students through a myriad of planned and impromptu events. Here are some of the staples:

Around the World: 

A roughly 12-hour boat race that involves miles of paddling. The fact that students are already hallucinating due to sleep deprivation adds a level of difficulty and humor. The winning crew gets some sleep, for as they say in BUD/S, “it pays to be a winner.” The crews, however, are alone during this event, and the break from the unforgiving gaze of the instructors is a welcome one.

Mud Flats:

About 15 hours of various events that take place in a mudflat close to the BUD/S compound. The mud invades every part of the students’ bodies, encasing them in cold clay.

Base Tour: 

A miles-long run around the BUD/S compound while carrying 300-pound rubber boats on their heads. Students often lose hair where the boats rub their scalp.

After Wednesday night, instructors ease on the throttle — without telling the students, of course. Instructors often threaten classes with additional days of Hell Week because of poor performance. But at some point, both the instructors and students know that no one is quitting anymore.

It usually falls to the BUD/S commanding officer, a SEAL captain, to secure Hell Week on Friday.

In First Phase, students wear white shirts. But after completing Hell Week, they get the coveted brown shirt that distinguishes them from those who haven’t completed the hurdle.

“The intent behind Hell Week’s craziness is to get down to a man’s true stuff — his true identity. Only when you have barely slept for freaking five days plus, doing crazy evolutions throughout this whole time while being wet and sandy, do you reveal your true colors,” a former enlisted SEAL with time as an instructor told Insider.

It is fairly common for students to finish Hell Week with serious injuries, such as dislocated shoulders and broken ankles, legs, and wrists. Extreme chaffing is another one.

Rolling back in training isn’t uncommon. Students often get rolled back because of medical or performance issues. It’s considered a small point of pride — and luck — in the community for a student to finish with his original class. But it’s possible to get roll backed even after finishing Hell Week.

For example, if a student fails the four allotted attempts for the Pool Comp test in Second Phase, he can be rolled back to Day One of BUD/S and have to redo Hell Week. (Instructors have sometimes waived Hell Week for two- or three-time rollbacks.)

“Hell Week boils down to essentially one thing — trust. Can I trust you to perform under the most extreme circumstances? Are you still a team player when everything around you is going up in flames? Hell Week does a pretty good job at revealing if a person falls into that box,” added the former SEAL instructor. “It is a gut check, plain and simple. We break them down to see if it’s worth building them up.”

After Hell Week, those who have survived go through what it’s called the “Walk Week,” essentially a low-intensity week with active recovery activities that offer students an opportunity to begin healing.

After all, another four-mile timed run is waiting just around the corner.

This article was written by Stavros Atlamazoglou and originally published on Business Insider.