I stared out into the cloudless blue Cretaceous sky. "I don't see anything coming."
"I wanted to get here early," Rachel said. "There'll be more time travelers soon, though. The last day of the Age of Dinosaurs has got to be a hot-ticket destination."
"No, I mean I don't see any sign of the Chicxulub impactor—the comet or meteor or whatever it is that's supposed to be about to smash into the Earth. Shouldn't we be able to see it from here?"
Rachel squinted into the sky. "You'd think. It certainly is a mystery."
"So how do we know we came to the right time?"
"Stratigraphy, old son. Way back in the twentieth century Alvarez père et fils realized that a thin boundary of clay marking the boundary between the Cretaceous and the Tertiary was unnaturally rich in iridium, no matter where in the world you examined it. Iridium is a lot more common in comets and asteroids than it is on Earth. So they postulated something big hit the planet, burned the forests, and killed the dinos—probably in the long nuclear winter afterwards."
"No need to worry. We'll only be here long enough to snap a few photos. We'll cut out before the destruction actually reaches us."
"Good. Hey! Isn't iridium the same stuff that's used to power our time machines? You don't suppose—?"
"No, no, of course not," Rachel said dismissively. "Oh, yes, certainly, there's a chance of chronodestablization. But it's very slight. Only one in ten thousand jaunts destabilizes. That's certainly a risk worth taking."
"Yeah, but if it did, it would go off with the force of an atom bomb. That's what they told me in orientation, anyway."
"Yes, but a single bomb wouldn't destroy the whole ecosystem! It would take—"
Her eyes bugged out. Time travelers were popping into existence everywhere. On the hilltops, by the banks of the rivers, in the mangrove swamps—everywhere. From every age possessing time travel they came in the hundreds, the thousands, the millions.… God knows how many there were in the entire world. More, possibly, than there were dinosaurs.
To every side of us light flashed into the sky and flashed and flashed and flashed, as one out of every ten thousand time machines destabilized and expressed itself as a thermonuclear explosion.