Now, at the bottom of the trench, the sub's custom-designed foam filling and the pressure-resistant shape of the "pilot sphere"—are helping protect Cameron from the equivalent of 8 tons pressing down on every square inch (1,125 kilograms per square centimeter).

To return to the surface, Cameron flips a switch inside the DEEPSEA CHALLENGER to electromagnetically cast off the steel plates that weigh the sub down, as shown in a sequence illustrated above. The cork-style ascent from Challenger Deep is estimated to take about 90 minutes.

If the weights don't drop, Cameron will be stuck at the bottom of the ocean with a very limited oxygen supply.

With the last few weeks, Cameron and his team have successfully completed untethered test dives in the submersible off Papua New Guinea and plan to conduct more tests near Guam (map) before attempting the Challenger Deep descent.

Cameron is so confident in his star vehicle that he started mulling sequels even before the trench dive.

Phase two might include adding a thin fiber-optic tether to the ship, which "would allow science observers at the surface to see the images in real time," he said.

Ultra-low frequency radio waves penetrate water for thousands of miles. No good for video, but fine for text and slow-scan pictures.

To return to the surface, Cameron flips a switch inside the DEEPSEA CHALLENGER to electromagnetically cast off the steel plates that weigh the sub down, as shown in a sequence illustrated above. The cork-style ascent from Challenger Deep is estimated to take about 90 minutes.

If the weights don't drop, Cameron will be stuck at the bottom of the ocean with a very limited oxygen supply.