Known more commonly as a “rebreather,” the Emerson closed-circuit diving rig functions differently than typical Scuba diving gear. Scuba—an acronym for “self-contained underwater breathing apparatus”—expels exhaled carbon dioxide into the surrounding environment. The rebreather, on the other hand, recycles expelled carbon dioxide, allowing the user to breathe any unused oxygen that’s been exhaled. Rebreathers are useful for covert or military operations because they do not expel gas into the water—unlike Scuba, they do not create noticeable trails of bubbles in the user’s wake.

This amphibious nickname applies to members of the Navy (especially the SEALs) or Marine Corps who are highly trained in Scuba diving and underwater tactical engagement. Military operations conducted in this environment typically involve combat duty or defense duty (such as demolition, mine defusal, etc.).

The SEALs—the United States Navy Sea, Air, and Land Teams—are the US military’s premiere special forces, trained for combat in all climates and conditions. They are typically considered the elite of the elite. Formally organized in 1962, SEAL teams trace their origins back at least to World War II, when various forces trained in special amphibious combat techniques to successfully make invasive beach landings on coastlines held by Axis forces.