Sixty-five million years before that, in the moist age of the dinosaurs, this place was ocean floor. The plains of the Gulf of Mexico were flooded up to the foothills of the Sierra Madre Oriental. The shallow Yucatan Peninsula itself lay under nearly a hundred meters of water. The sediments that would later form Cuba and Haiti were part of the deep seafloor, yet to be lifted by fault movements to the surface.

In an age dominated by warm shallow seas, drowned Chicxulub was an unremarkable place. But it was here that a world would end.

Chicxulub is a Mayan word, an ancient word coined by a lost people. Later, when the Mayans were gone, nobody would know for sure how it translated. Local legend said it meant the Devil’s Tail.

In its last moments the comet flew in from the southeast, passing over the Atlantic and South America.

In bright, shallow waters the huge ammonite cruised.

This sea-bottom hunter, the size of a tractor tire, looked something like a giant snail, with an elaborately curved spiral shell from which arms and a head protruded cautiously. As it had grown, it had extended its shell’s spiral structure, gradually moving from one chamber outwards to the next; now the linked, abandoned chambers were used for buoyancy and control.

The ammonite moved with surprising grace, its upright spiral cutting through the waters. And it scanned its surroundings with wide intelligent eyes.

The sunlit sea was crowded, translucent, full of rich plankton. Some of the creatures here — oysters, clams, many species of fish — would have been familiar to humans. But others would not: there were many ancient species of squid, the ammonite itself — and, dimly visible as shadows passing through the blue reaches of the deeper ocean — giant marine reptiles, mosasaurs and plesiosaurs, the dolphins and whales of the age.

As the daylight gathered, more of the ammonite’s kind were rising, to hang like bells in the translucent water.

But the ammonite spotted movement on the seabed. It descended quickly, sensory tentacles pushing out of its shell. By sight and feel it quickly determined that the scuttling, burrowing thing in the gritty sand was a crab. More arms slid out of the shell and wrapped around the crustacean, tiny hooks on each arm helping to secure their grip. The crab was pulled easily away from the soft seabed. A heavy birdlike beak protruded, and the ammonite bit through the crab’s shell, between its eyes. It injected digestive juices into the shell, and began to suck out the resulting soup.

As particles of meat diffused in the water, more ammonites came sliding in.

But the ammonite with the crab saw a shadow moving above, a shadow with a snout and fins, silently sliding, rapidly resolving. It was an elasmosaur; a marine reptile, a kind of plesiosaur with an immensely long neck. Abandoning its kill, the ammonite ducked into its shell. The opening in the shell was immediately sealed off with a heavy cap of hardened tissue.

The elasmosaur fell on the ammonite, pushed its shell over, and clamped its powerful jaws around the narrowest part of the spiral. But it could not break through. After breaking a cluster of teeth, the elasmosaur dropped the shell, letting it drift back to the ocean floor. Frustration and pain seethed in its one-dimensional awareness.

The ammonite had endured violent shaking, but it was safe in its armored home.

But one immature ammonite had not been so wary. It tried to flee, its jets pushing it this way and that.

The elasmosaur took its consolation kill well. Its teeth sliced expertly across the spiral shell at the place where the body was attached to the inner surface. Then it shook the shell hard until the ammonite, still alive, tumbled out into the water, naked for the first time in its life. The fish-lizard took its prize in a single gulp.

Now the elasmosaur spotted a cloud in the water. It plunged in without hesitating.

The cloud was a shoal of belemnites, thousands strong. The little squid had gathered for protection, and their defensive systems, of sentries and ink and shimmying, deceptive movements, were usually effective even against predators as fast as this elasmosaur. But they had been caught out by this creature’s angry lunge. They darted away, venting ink furiously at the immense invader, or even leaping out of the ocean altogether and into the comet-bright air. Still, hundreds of them died: each a pinpoint of awareness, each of them in its way unrepeatable and unique.

Meanwhile, cautiously, the crab-killer ammonite had opened its shell once more. A tube of muscle protruded from the opening, and a high-pressure stream of water pulsed out, jetting the ammonite up and into the blue waters. It had lost the crab. But no matter. There was always another kill to make.

So it went. It was a time of savage predation, in the sea as on land. Mollusks hunted ammonites, boring through shells, poisoning prey animals, and firing deadly darts. In response, bivalves had learned to bury themselves deep in sediment, or had evolved spines and massive shells to deter attackers. Limpets and barnacles had forsaken the deep sea, colonizing shallow environments on the shore where only the most determined of hunters could reach them.

Meanwhile, the seas teemed with predatory reptiles. Carnivorous turtles and long-necked plesiosaurs fed on fishes and ammonites — as did pterosaurs, flying reptiles who had learned to dive for the riches of the ocean. And huge, heavy-jawed pliosaurs preyed on the predators. Measuring some twenty-five meters long, with jaws alone some three meters long, their sole stratagem to rip and shake their prey apart, the pliosaurs were the largest carnivores in the history of the planet.

The rich Cretaceous oceans teemed, enacting a three-dimensional ballet of hunter and hunted, of life and death. It had been so for tens of millions of years. But now a bright light was building above the glimmering surface of the ocean, as if the sun were falling from the sky.

The ammonite’s eye was drawn upwards. The ammonite was smart enough to feel something like curiosity. This was new. What could it be? Caution prevailed: Novelty usually equated to danger. Once more the ammonite began to withdraw into its shell.

But this time even its mobile fortress could not protect it.

The comet punched through Earth’s atmosphere in fractions of a second. It blasted away the air around it, blowing it into space, leaving a tunnel of vacuum where it had passed.

The ammonite was trapped right under the comet’s fall. It was as if a great glowing lid closed across the sky. Its substance immediately vaporized, the ammonite died. So did the belemnites. So did the elasmosaur. So did the oysters and clams. So did the plankton.

The ammonites had stalked the oceans of the Earth, spawning thousands of species, for more than three hundred million years. Within a year, none of them would be left alive, none. Already, in these first fractions of a second, long biographies were being abruptly terminated.

The few dozen meters of water offered the comet nucleus no more resistance than the air. All the water flashed to steam in a hundredth of a second.

Then the comet nucleus hit the seabed. It massed a thousand billion tons, a flying mountain of ice and dust. It took two seconds to collapse into the seabed rocks, delivering in those seconds the heat energy released by all of the Earth’s volcanoes and earthquakes in a thousand years.

The nucleus was utterly destroyed. The seabed itself was vaporized: rock flashed to mist. A great wave pulsed outward through the bedrock. And a narrow cone of incandescent rock mist fired back along the comet’s incoming trajectory, back through the tunnel in the air dug out in the comet’s last moments. It looked like a vast searchlight beam. Around this central glowing shaft, a much broader spray of pulverized and shattered rock, amounting to hundreds of times the comet’s own mass, was blown out of the widening crater.