The world was jarred by the loss of the five men aboard the Titan submersible that imploded last week as it went down to explore the Titanic shipwreck. The depth of that dive was around 13,000 feet, or about 2.5 miles, or four kilometers. The deep sea is dangerous whether you are in a submersible or a submarine. Wikipedia has an extensive list of incidents and accidents involving underwater transport, many of them deadly. But how deep was the deepest successful underwater rescue?
That would be 1,575 feet. Roger Chapman and Roger Mallinson were laying transatlantic cable in the submersible Pisces III off the coast of Ireland in 1973. After their shift, the submersible was towed up by its cable, but at the surface, the tow line tangled with a hatch, popped it open, and water flowed in. The submersible sank again, snapping the tow line when it reached its limit, then fell to the ocean floor. The two men inside had no lights, but by a minor miracle, they had 64 hours of oxygen left. They also had a support ship above them with other submersibles. But those had 1973 technology, and weren't built to rescue people from the ocean floor. By the time the men returned to the surface, they had been in the submersible for 84 hours. Read about the complicated rescue of the Pisces III at Amusing Planet.
(Image credit: United States Navy)