If you have any kind of hunger for real-world adventure or if you’re any kind of gearhead, James Cameron‘s Deepsea Challenge 3D (Disruptive, 8.8) is an essential — a fascinating, highly intelligent, smartly assembled doc (co-directed by John Bruno, the late Andrew Wight and Ray Quint). Definitely catch it in IMAX if you can. The subject, of course, is Cameron’s solo seven-mile descent to the bottom of the Mariana trench — 35,787 feet — on 3.26.12. He did this inside a privately-designed, funded and constructed submarine called the Deepsea Challenge, and all the time I was watching the doc I was saying to myself, “Amazing, I love this, Cameron and his team are so hard-core…but why the fuck is Jim making three Avatar sequels? Isn’t a trilogy enough, for God’s sake?” You get the idea that he’s making three because he wants a lot more money — i.e., our movie money — so he can self-fund even more undersea explorations.

I found it irksome that the doc doesn’t break down the costs, and particularly how much of it Cameron paid out of his own pocket along with donations from the National Geographic guys and the Woods Hole Oceanographic Institution. He’s apparently worth around $900 million so how much did he personally fork over? C’mon, guys…spit it out.

The size of the pilot sphere that Cameron occupied during the dive is very tight — only about 43 inches long. It looks a tiny bit smaller than the area that the original Mercury astronauts had to squeeze into inside those little space capsules. If you’re the least bit claustrophobic (as I am) it’s somewhat uncomfortable to even think about this, much less watch it happen.

The sub is mostly composed of a specialized structural syntactic foam called Isofloat, which is capable of submitting to the massive water pressure at 6.8 miles of depth. But what’s the shiny green surface made of? And what’s that sudden cracking noise that we hear when Cameron is doing a test dive? And why does the temperature suddenly rise into the 90s during another test?

The finale lacks a certain something as the bottom of the trench is as dead and cold as the surface of the moon. Cameron is the third guy in world history to go all the way down there, but…I hate to say this but to what end? Not to diminish the achievement — it’s a thrill to see Cameron and his team work and try so hard and finally get there. But what does it matter, really, except to say “we did this”?