Historic underwater tests that pushed human survival to the limit

As SeaLab II descends toward the seabed, crews discover a rapid helium leak across all four clamps, forcing the commander to make a high stakes call to race to the bottom.

Emergency Helium Leak During SeaLab Lowering

From outer space to 200 feet beneath the waves.

Astronaut Scott Carpenter prepares for a record 30-day undersea stay, swimming down to enter SeaLab II through an open bottom hatch held clear purely by internal gas pressure.

Mercury Astronaut Enters Ocean Floor Habitat

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First conversation between the bottom of the sea and outer space

Inside Sealab II, Scott Carpenter establishes communication with Gordon Cooper aboard Gemini 5, marking humanity's first live chat between the deep ocean and outer space.

First Call Between the Ocean Floor and Space

You cannot strike a match in an underwater habitat

An aquanaut celebrates his birthday underwater, but cannot light any candles because the specialized helium atmosphere lacks enough oxygen for combustion.

Why Matches Will Not Light Inside Sealab

Even with a fatal wound, you cannot return to the surface

After being stung by a venomous scorpionfish, an aquanaut must be treated inside the habitat because emergency surfacing would trigger fatal decompression sickness.

Why Injured Aquanauts Cannot Surface

Singing 200 feet underwater on pure helium

Inside the Sealab II habitat, astronaut and aquanaut Scott Carpenter plays the ukulele and sings Goodnight Irene in a high-pitched helium voice. The clip captures a lighthearted moment of living in a pressurized synthetic atmosphere.

Astronaut Sings with a Helium Voice 200 Feet Underwater

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Footage highlights Tuffy the porpoise, trained to respond to acoustic beacons, ferry supplies, and guide divers back to safety during deep-water operations. It showcases early military research into marine mammal communication and teamwork.

Meet Tuffy: The US Navy's Underwater Rescue Porpoise

While decompressing inside a chamber, Commander Scott Carpenter speaks via telephone with President Lyndon B. Johnson. Because of the pressurized gas mixture, the Commander reports to the Commander-in-Chief in an unmistakable chipmunk pitch.

Aquanaut Takes a Call from the President in a Helium Voice

One mistake turns this capsule into a coffin

Ten divers are hoisted to the surface while still pressurized to ocean floor depths, where any single mechanical error would prove immediately fatal.

The Capsule That Could Become a Coffin

Inside the decompression chamber, an aquanaut reports worsening pain spreading from his knee to his hip, prompting the flight surgeon to order immediate recompression.

Severe Bends Strike Aquanaut During Decompression

[04:52] Emergency Helium Leak During SeaLab Lowering
[06:54] Mercury Astronaut Enters Ocean Floor Habitat
[09:11.260] First Call Between the Ocean Floor and Space
[13:00] Why Matches Will Not Light Inside Sealab
[13:37] Why Injured Aquanauts Cannot Surface
[17:06] Astronaut Sings with a Helium Voice 200 Feet Underwater
[17:36.000] Meet Tuffy: The US Navy's Underwater Rescue Porpoise
[19:58] Aquanaut Takes a Call from the President in a Helium Voice
[24:22] The Capsule That Could Become a Coffin
[25:07] Severe Bends Strike Aquanaut During Decompression

Historic underwater tests that pushed human survival to the limit

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