Deep Wizardry-wiz 2 by Diane Duane

bottom isn’t much deeper than a hundred feet within that distance. But then—you see a lot of contour lines packed closely together? That’s the edge of the Continental Shelf. Think of it as a cliff, or a mesa, with the North American continent sitting on top of it. Then there’s a steep drop—the cliff is just a shade less than a mile high—“
    “Or deep,” Nita said.
    “Whichever. About a five thousand foot drop; not straight down—it slopes a bit—but straight enough. Then the sea bottom keeps on sloping eastward and downward. It doesn’t slope as fast as before, but it goes deep-some fifteen thousand feet down; and it gets deeper yet farther out. See where it says ‘Sohm Abyssal Plain’ to the southeast of the Island, about six or seven hundred miles out?”
    “It has ‘the Crushing Dark’ underneath that on our map,” Nita said. “” that the whales’ name for it?”
    “Right. That area is more like seventeen, eighteen thousand feet down.”
    “I bet it’s cold down there,” Kit muttered.
    “Probably. Let me know when you get back,” Tom said, “because that’s where you’re going.”
    Nita and Kit looked at each other in shock. “But I thought even submarines couldn’t go down that far,” Nita said.
    “They can’t. Neither can most whales, normally—but it helps to be a wizard,” Tom said. “Look, don’t panic yet—“
    “Go ahead! Panic!” screamed Picchu from somewhere in the background. “Do it now and avoid the June rush! Fear death by water!”
    “Bird,” Carl’s voice said, also in the background, “you’re honing for a punch in the beak.”
    “Violence! You want violence, I’ll give you violence! No quarter asked or given! Damn the torpedoes, full speed ahead! Don’t give up the AWWWKI”
    “Thanks, Carl,” Tom said, as silence fell. “Where were we? Oh, right. You won’t just be going out there and diving straight down. There’s a specific approach to the Plain. Look back closer to the Island, and you’ll see some contours drawn in dotted lines—“
    “Hudson Channel,” Nita said.
    “Right. That’s the old bed of the Hudson River—where it used to run a hundred thousand years ago, while all that part of the Continental Shelf was still above water. That old riverbed leads farther southeast, to the edge of the Shelf, and right over it ... there was quite a waterfall there once. See the notch in the Shelf?”
    “Yeah. ‘Hudson Canyon,’ it says—“
    “The Gates of the Sea,” said Tom. “That’s the biggest undersea canyon on the East Coast, and probably the oldest. It cuts right down through the Shelf. Those walls are at least two or three thousand feet high, sometimes four. Some of the canyons on the Moon and Mars could match the Hudson —but none on Earth. And for the whale-wizards, the Gates have become the traditional approach to the Great Depths and the Crushing Dark.”
    The thought of canyon walls stretching above her almost a mile high gave chills. She’d seen a rockslide once, and it had made her uneasy about canyons in general. “Is it safe?” she said.
    “Of course not,” Tom said, sounding cheerful. “But the natural dangers are Carl’s department; he’ll fill you in on what precautions you’ll need to take, and I suspect the whales will too.”
    “ ‘Natural dangers,’ “ Kit said. “Meaning there are unnatural ones too.”
    “In wizardry, when aren’t there? This much I can tell you, though. New York City has not been kind to that area. All kinds of things, even unexploded depth charges, have been dumped at the head of Hudson Canyon over the years. Most of them are marked on your map; but watch out for ones that aren’t. And the city has been dumping raw sewage into the Hudson Channel area for decades. Evidently in the old days, before people were too concerned about ecology, they thought the water was so deep that the dumping wouldn’t do any harm. But it has. Quite a bit of the sea-bottom life in that area,