Imagine a place where every single inch of your skin is under eight tons of weight. That’s not some planet in a galaxy far, far away: it’s the bottom of the bottom of the ocean. At its deepest point the Mariana trench is 36,000 feet (11,000m) deep, and water pushes from all sides with 16,000 psi (pounds per square inch) — over 1000 atmospheres. That spot, called the Challenger Deep, is where James Cameron plans to be traveling very soon. His secretively constructed submersible, the Deepsea Challenger, is designed to test the limits of the oceans and allow him to see, record, and bring samples back from its most remote region.

What does it take to build, operate, and safely retrieve a submersible — many in the industry reserve the term submarine for military vessels — from those depths? Cameron and his team haven’t been saying much, but analyzing past efforts by Cameron and others, plus what is known publicly, is enough to take you through the tech needed to make Cameron’s vision a reality. Dealing with the intense pressure, tricky buoyancy issues, keeping electronics working, and of course creating memorable video from the seabed top the list of challenges.

Pressure

First and foremost of the challenges is the mind-numbing amount of pressure: 16,000 psi. That’s like having three entire cars resting on your thumb. And three more on your index finger, and so on. Plenty of materials are strong enough not to be crushed under the pressure, but not very many can be formed into a chamber large enough to fit a human inside with a way to see out.

Classic submersibles, like the Trieste — the only manned vehicle to have ever reached the Challenger Deep in the Mariana Trench before — had the tiniest of windows set into massively thick hulls. As Liz Taylor of DOER, which builds undersea craft for researchers says, scientists really want to see. Otherwise, of course, it’d be simpler just to send down an unmanned craft with some cameras. DOER has taken the radical approach for its proposed deep sea vehicle by planning a single, 4-foot in diameter cast sphere of glass. Taylor contends that by having the glass made out of a single piece, it will be strong enough to withstand the pressure. Of course it will still need a hatch opening and some way for all the sub’s control systems and camera gear to communicate from inside to outside.

Since the DOER project is still on the drawing board, it’s hard to know whether it will work out, but existing designs have taken a more conservative approach to windows. Richard Branson’s group has created a “half-dome” of borosilicate (essentially a fancy version of what we all know as Pyrex) that provides an impressive visual range for the single pilot laying in the vehicle. Unfortunately the viewing window on their submersible cracked under a “mere” 2200 psi, casting doubt on how far down the craft will be able to travel without a major rework. As recently as last year, Cameron speculated that Branson was ahead in the X Prize “race to the bottom,” but now the tables have turned and Cameron is fast closing in on the prize.

It is likely that Cameron, being brave but not entirely foolish, has taken a more conservative approach, using a smaller porthole that will prove safer in the long run. Even simple portholes are made complicated by the pressure. Instead of a straightforward “plate” like we’ve all seen on cruise ships or in movies featuring diving bells and shallow water subs, deep sea craft typically use a conical shape, with the outer surface of the window being larger than the inner. That distributes the pressure over a broader area of the supporting hull and makes it possible to build windows that are both safer and more secure.

The design of the pressure vessel, also called a pressure hull, is also a feat of engineering. Pure steel chambers are strong, but traditional steels tend to be much too heavy to be floated easily. Titanium is a better choice, but also heavier than is ideal. Ceramic offers a lightweight and very strong option, but is very brittle and therefore subject to cracking. The Nereus — an unmanned craft which Woods Hole sent into the Mariana Trench — had all of the openings in its pressure vessel in the titanium half of its hybrid ceramic-titanium model to avoid further stressing the ceramic piece. Some research has been done into using a ceramic pressure vessel with a glass dome at one end, but it is not clear if any of it is ready for use on the Challenger. Cameron’s team has now said that the actual “pilot sphere” is a 43-inch steel unit.

There is another equally deadly, but less obvious, threat from pressure. If a low-pressure area gives way, the resulting force of water at 16,000 psi rushing to fill the space is the equivalent of a large explosion — in this case actually an implosion. If it were to happen, it is likely the entire vehicle, and its crew, would be destroyed. That is a big reason that deep sea craft are best designed with just a single low-pressure chamber for the crew, while all the other gear is built to simply survive with the pressure — creating some other design challenges covered later in this article.