Here's the scenario: After suiting up with diving knives and silenced assault rifles, a team of three Navy SEALs on a submarine prepare to head to shore for a sneak attack. They put on their scuba gear and climb into a vessel that shaped like a torpedo and not much bigger than a shower. Powered by a single rear propeller, it deploys from the host submarine. After hours of slow, calculated movement through water too shallow for any submarine, radar indicates the SEALs have reached shore. Still underwater, they slide back the top canopy of their vessel and swim the last stretch to the beach under cover of night.

The key tool here is the SEAL Delivery Vehicle (SDV), the modern version of what's essentially a tube with a propeller stuck on the back. It can be as compact as needed, sized to fit just one Navy SEAL or as many as six. These craft are typically “free-flooding” vessels, which means they're filled with water. The soldiers inside breathe through their own scuba tanks or from on-board oxygen reservoirs. The first guys to lock themselves inside these coffin-sized machines were Chuck Yeager-grade brave.

The idea of a scaled-down, maneuverable submarine has been around for decades. Full-sized subs can't operate properly in water shallower than 50 feet, so getting covert forces from ship to shore is tricky. Going in with scuba gear may require more oxygen than can be contained in a normal tank, and swimming in flippers for that long can leave even a hardened SEAL too exhausted to perform the mission.

Back in World War II, the British Office of Strategic Services Maritime Unit (OSS MU) made a vehicle called the “submersible canoe,” or "sleeping beauty." Soldiers trained with it and ran test missions, but it never saw live combat. In the early 1970s, small craft were used off the coast of North Vietnam for combat and reconnaissance. One, the Mark VII, was built with a fiberglass hull and components made with non-ferrous metals, to minimize the craft’s radar signature. Power to the rear propeller came from two rechargeable silver-zinc batteries.

As the vessels now used by SEALs evolved, updates included Doppler navigation systems, sonar, and docking systems that reunited the unit with its host submarine once its occupants have filed out. A dry dock shelter lets soldiers move from the sub to the water, then get into the vessel.

The modern version of these “underwater swimmers” was officially commissioned in 1983 by the Naval Special Warfare SEAL Delivery Vehicle Teams. The then-new Mark VIII, which succeeded the Mark VII in the early 1980s, was designed to be much larger than its predecessors so it could move more cargo and more personnel, propelled by rechargeable batteries. The Mark VIII and its successors saw combat in Operation Desert Storm, and were used to secure offshore oil terminals during the most recent Iraq war.

JF Defence

If you've caught any episodes of Michael Bay's TV show The Last Ship, you'll recognize the Torpedo SEAL, made by James Fisher Defence. The vessel, which is not currently used by the US Navy, can be deployed from a NATO-standard 533mm torpedo tube, or air-dropped from a helicopter. Propulsion comes from a lithium polymer battery that gets it to a top speed of 4 knots, or just under 5 mph. JF Defence says its "deliveries [are] often named as non-disclosed," but that their six-man SEAL Carrier has been delivered to Sweden's Navy.

The most recent variation on the SDV is the new the Shallow Water Combat Submersible (SWCS). Alabama-based Teledyne Brown Engineering won the $383 million contract in 2011 to design and build this new craft. We called them up, but Teledyne "respectfully decline[d] the opportunity to be interviewed." They must be doing something cool.