A FEW DAYS AFTER I return home, a DVD arrives from Cassell. It reads HUMBOLDT SQUID CLIPS, and the letters are burned into a picture of Cassell sitting in a panga with a Humboldt across his lap. It's not just any squid. It's thick and muscular, perhaps twice the size of anything we saw.

"OK," I say to myself, and cue up the DVD. I scroll my way past clips of Cassell pimping in his Star Wars squid armor for the Discovery Channel and telling spooky tales to get people squirming in their Barcaloungers. I check out a sequence called "Attacks," in which flashing squid take runs at Cassell, their arms grating across his camera gear as he grunts and pants for breath. Finally, I find a segment called "Giants" and settle in.

Cassell is in a pale-green underwater world, a couple hundred feet down, and directly in front of him is an utterly massive squid. (Cassell estimates that it was about eight feet long and perhaps 200 pounds.) Its skin is pocked by scars from numerous battles, and its body appears unusually heavy. Dozens of other large squid are swimming nearby, flashing their eerie messages, and none of them seems the least bit intimidated by Cassell. Indeed, they repeatedly jet in to grab at him. Sometimes the probes come head on, the squid lining up like darts and then bull-rushing him with a flare of arms and tentacles followed by the scrape of sucker teeth on his armor. Sometimes there are sneak attacks from behind, signaled by grunts and yelps from Cassell as he tries to free himself. During one attack, you can hear Cassell laughing maniacally before finally conceding an "Owww."

He's in a world that would be terrifying to the average diver. But he's loving it. The mega-squid, which Cassell will later dub Scar, appears again and again to eyeball Cassell up close and probe him with its tentacles. Scar seems almost imperious in his disdain for this human invader, and Cassell will later write, "It occurs to me that this might well be the first encounter of its kind for both species. I can only describe it as a dance. A dance of peace, curiosity, and discovery."

Who knows how well Cassell really understands these animals and whether a human can ever really dance with a squid. But Cassell is a romantic. What's important to him is that in those moments beneath the Sea of Cortez, he transcends the human world and dives his way into an undersea realm that is wild, often brutal, and ruled by a spooky alien species. The monsters are really there; you just have to dive deep enough to find them. And that's something most of us will never do.