The toughest US Army Special Forces course takes place underwater. Here's why it has to be brutal.

• There is a very small Army community that goes through the Combat Diver Qualification Course.
• Participants, mostly Green Berets, are tested on their diving know-how and ability to perform under pressure.

When someone talks about frogmen or combat divers, most people immediately imagine Navy SEALs, the dedicated maritime component of the US special-operations community.

But SEALs aren't the only ones who do combat diving. There is a very small Army community, mostly made up of special operators, that goes through the Combat Diver Qualification Course to become combat divers.

It's arguably the toughest course in the Army, and as the focus shifts from ground wars in the Middle East to competition around the world, the skills it teaches are likely to be in higher demand.

Here there be monsters

Established in the 1960s, the Combat Diver Qualification Course takes place in Key West, Florida.

An important feature of the course is its adaptability to current or future threats, and it has gone through many variations. It typically lasts between four and six weeks. Only a select few attend.

"The majority of students that attend CDQC are special operators from the Green Berets, Special Mission Units" — such as Delta Force or SEAL Team 6 — "Rangers, and occasionally soldiers from infantry and reconnaissance units. In addition to active-duty service members, ROTC cadets from universities around America and West Point cadets also can attend," a retired Green Beret who was a course instructor told Insider.

The course is divided into two phases.

During the first phase, instructors focus on physical fitness, equipment familiarization, and teamwork. Next, students are paired in groups of two and become "dive buddies."

The first phase ends with the dreaded pool stress events. Instructors test the students' coolness, proficiency, and ability to follow procedures that could potentially save their live or their buddy's lives down the line.

Students need to pass standard events: the 50-meter underwater swim, retrieval of a 20-pound object from the bottom of the pool, and the two-minute water tread.

The students also have to excel academically.

"Academic classes covering subjects such as diving physics, dangerous marine life, diving physiology, diving injuries, and decompression are taught and tested. Students learn the use of open-circuit SCUBA equipment, which is used during search dives, ship hull searches, and open-water deep dive," the retired Special Forces operator said.

Students also learn the all-important decompression procedures. Incorrect decompression protocol can cause paralysis and even death as dissolving gases become bubbles inside the body's tissues, also known as "divers' disease."

The second phase covers the tactical aspects of combat diving. In particular it focuses on the Mark 25 Draeger oxygen rebreather, a closed-circuit underwater breathing apparatus that doesn't emit bubbles, enabling special operators to swim undetected.

Students also conduct several underwater dives to hone their navigation skills and work with small boats and aircraft, such as helicopters, as different insertion and extraction methods throughout the course.

The course ends with a full mission profile that includes insertion by air and a closed-circuit dive.

The Army's toughest school

Members of the Army special-operations community generally agree that the Combat Diver Qualification Course is the hardest course.

All course candidates have to go through a pre-course selection, the Maritime Assessment Course, to even qualify for the Combat Diver Qualification Course. Many of the special operators who attend the course fail or drop out, and the majority of them have already been assigned to a special-operations unit.

The course's difficulty is measured in three different ways, "the primary being adaptability in a water environment while in stressful situations — this being the one aspect which students can't fully prepare themselves for until they are in the course," the retired Green Beret and former instructor told Insider, speaking anonymously to avoid compromising ongoing work with their former unit.

"Physical fitness is evaluated and emphasized throughout the entire course, making it one of the most difficult courses in the military. Thirdly, students' academic abilities are monitored and tested throughout the course," the retired Green Beret added.

Besides the level of toughness, the Combat Diver Qualification Course is also dangerous. The course has stringent safety precautions, but deaths have happened. Two Special Forces operators died while attending the school in late 2021.

Those who do graduate earn the coveted Combat Diver insignia and rejoin their units. In the case of Green Berets, who make up most of the students, they join a combat dive team that's part of their Special Forces group.

A cadet recently made history by becoming the first woman soldier to graduate the course and become a combat diver. A woman going through an Air Force special-operations pipeline has also graduated the Army course.

The Army combat diver community has often been overlooked and underused during the past two decades, as there was little need for combat divers in the wars in the Middle East.

As great-power competition with near-peer nations, namely China and Russia, heats up, combat divers are likely to be called on more to apply their capabilities in the waters of Europe, Asia, and around the world.

Stavros Atlamazoglou is a defense journalist specializing in special operations, a Hellenic Army veteran (national service with the 575th Marine Battalion and Army HQ), and a Johns Hopkins University graduate.