A rough depiction of the Chicxulub Crater impact zone.

The Big One

An asteroid wiped out the dinosaurs 66 million years ago. Now geologists are drilling into the crater left behind to learn how it happened—and what it means for life on Earth today.

By Rose Cahalan
Photos by Anna Donlan

It's just after 9 a.m. and the sun is already merciless. I’m standing at the bow of a small boat in the Gulf of Mexico, a few miles offshore from the sleepy Mexican port town of Progreso. The occasional blast of sea spray is a welcome respite in the 95-degree heat, although the salty droplets evaporate quickly, leaving a sticky film on my face, and I can feel my skin toasting despite my best efforts with sunscreen. The two geologists next to me—Axel Wittmann, of Washington University in St. Louis, and Mario Rebolledo Vieyra, of the Cientro de Investigación Científica de Yucatán, are practically bouncing on their heels with excitement, shielding their eyes as they point out a tiny black dot emerging on the turquoise horizon. “Look, there it is!” Wittmann exclaims.

A view of the Liftboat Myrtle from the Linda crew boat.

As we sail closer, the dot grows into a mirage: a boat floating high above the water. It seems as impossible as a flying car. But in fact, this 200-ton vessel, the Liftboat Myrtle, really is suspended 50 feet in the air, propped up by three legs that sink down into the muddy seafloor below.

Our boat lists from side to side with the waves as a crane operator sends down a small basket suspended on a wire. The basket, about the size of a kiddie pool, holds four people and is the only way on and off the Myrtle. There are no doors or straps—you just step in, grab a rope, and pray. My breath catches as we lift off, and at first I feel like a stuffed toy caught in an enormous claw machine. But the view is undeniably spectacular as the basket flies over both boats, a curious seagull flapping a few feet from our faces. With a slight bump, we touch down on the Myrtle, where several people in red jumpsuits and white safety helmets have lined up to greet us. One of them is a man with shoulder-length brown curls and a mug of black coffee that, I’ll learn later, he is rarely seen without. He carries himself with the regal bearing of a competitive medieval jouster, which he is. He extends a hand, cracks a smile, and says, “I’m Sean. Welcome aboard!”

A professor in UT’s Jackson School of Geosciences, Sean Gulick is co-leading one of the most far-reaching scientific research projects in decades. For two months this spring, he and 11 other scientists traveled from around the world to live and work on the Myrtle. They were there to drill into the Chicxulub crater—the underground footprint of a giant asteroid that wiped out 75 percent of all life on Earth, including the dinosaurs, when it blasted into the planet 66 million years ago.

Scientists have been studying the crater since the 1980s, but this $10 million expedition, which took 18 years to plan, is the most ambitious study yet. It’s also the first to drill it from offshore and the first to look at the crater’s peak ring, a discontinuous chain of mountains around the impact. Because the peak ring was formed by the impact, analyzing it will give scientists a better understanding of what happened when the asteroid hit and how life recovered afterward. While Earth has two other large impact craters, Chicxulub, which is 125 miles in diameter, is unique. “This is the best-preserved large impact on the planet,” Gulick says, “and it’s the only one tied to a mass extinction and the only one with a confirmed peak ring. This can’t be done anywhere else in the world.”

Scenes from the Chicxulub research expedition.

To understand why, you have to go back to a very bad day 66 million years ago. The sky would have darkened as a massive asteroid 10 miles in diameter approached the sea near the modern-day Yucatan Peninsula at a staggering 76,000 miles per hour. When it crashed into the Earth, the force of the asteroid’s impact was 100 million times stronger than an atomic bomb. But that was just the beginning. Continental shelves collapsed and mega-tsunamis roiled the oceans. The globe was ravaged by wildfires, volcanic eruptions, and earthquakes that would have rated 12-14 on the Ricter scale (the scale only goes to 10).

At all hours, you can hear the drill: a rhythmic humming and clanging that gets louder as you near the bow of the boat. Every minute counts on this expedition, because it costs thousands of dollars per hour to operate a facility like this in the middle of the ocean.

“Then you have all the ejecta, which raised the temperature of the planet,” Gulick adds. This is a rather gentle way of saying that there was a hellish rain of fire as pieces of Earth that were blasted out into space by the impact fell back to the surface. Some of that material, or ejecta, stayed in the atmosphere, blocking out the sun for months or years. Scientists aren’t sure how long this “nuclear winter” lasted. What is clear is that the entire planet was cold and dark for long enough to starve plants of sunlight and cut down on photosynthesis. The food chain collapsed, and only 25 percent of life survived, putting an abrupt end to the 160-million-year reign of the dinosaurs.

If all this is sounding like the plot of a bad Bruce Willis movie, you aren’t alone. Thirty years ago, many scientists scoffed at the idea that an asteroid could have killed the dinosaurs. “There was a lot of resistance initially,” says Chris Lowery, a UT postdoctoral researcher who participated in the Chicxulub expedition. “It may have seemed too cool to be true.” At the time, geologists favored the idea that the Earth changed due to more gradual forces, a school of thought called uniformitarianism, as opposed to catastrophic events. “But like most things in science,” Lowery says, “it turns out that it’s a little bit of both.”

Then, in 1977, geologist Walter Alvarez of the University of California, Berkeley stumbled on a layer of iridium—a metal rare on Earth, but common in asteroids—in the the Italian village of Gubbio. He and his father, Nobel laureate Luis Alvarez, and other collaborators published a paper in 1980 proposing that an asteroid, not gradual climate change, was to blame. Soon scientists had found iridium not just in Italy, but in hundreds of other sites around the world. Supporting evidence mounted over the years, with the biggest smoking gun being the 1991 discovery that Chicxulub was an impact crater, and now the theory is widely accepted. “I’d say we’re at a 95 percent consensus,” Gulick says. “But the way science works is that you say, ‘I’ve learned this, but now we have 10 new questions based on our original two.’”

Among those new questions: What exactly happened to the planet when the asteroid hit, how did it cause the mass extinction, and how did life bounce back afterward?

From left: Sean Gulick with his ubiquitous cup of coffee; the L/B Myrtle as seen from the Billy Pugh basket; DOSECC drilling manager Chris Delahunty passes the time next to playful directional signage created by scientists and crew.

Life aboard the Myrtle is hot, sweaty, and crowded. The boat is home to a rotating cast of 12 scientists, plus 21 more working on land, from a total of 11 countries. There's also a crew of sailors, deckhands, drill operators, and a cook. Six bunk beds are packed into each dorm room, but expedition manager Claire Mellett tells me her bunk, with its privacy curtain, is her favorite place on the boat. “It’s the only spot where you can find a moment to yourself,” she says. Everyone works 12-hour shifts, often more, and sleeps when they can. Pastries are served at 3 a.m. to boost morale.

In their scant free time, the researchers entertain themselves with something they call Fish Cam—a GoPro lowered into the ocean to record the tropical fish swimming next to the drill. No alcohol is allowed on the boat, a complaint that one thirsty scientist turns into a prank: He asks newcomers to name their favorite beer, prints off a photo of its label, tapes it over a soda can, and waits to see their gleeful reaction when they spot it on a shelf of Cokes in the mess hall. “We do get a bit weird out here,” Mellett laughs.

At all hours, you can hear the drill: a rhythmic humming and clanging that gets louder as you near the bow of the boat. Every minute counts on this expedition, because it costs thousands of dollars per hour to operate a facility like this in the middle of the ocean—and because time-traveling through 1,000 meters of the Earth’s history isn’t easy. New cores, or three-meter cylinders of rock, arrive on deck every 90 minutes. After the cores cool, researchers spring into action inside six shipping containers turned into miniature laboratories.

In one of these labs is Erwan Le Ber, a research associate from the University of Leicester in England. He uses a device called a multi-sensor core logger, which looks a little like a mini train track, to send each core through a series of tests for traits like gamma radiation and magnetic sensitivity. He’s logged 350 meters so far, but today he’s run into technical difficulties. “It has crashed four times in a row,” Le Ber says, with a rueful shake of his head. “We are still getting very good data.” Mishaps are business as usual when it comes to fieldwork. On the day of my visit, the drilling team is busy replacing a broken drill bit.

“By eliminating the dinosaurs, the impact paved the way for mammals to evolve. So it definitely had a role in the fact that we’re here today.”

In other labs, micropaleontologists are studying tiny fossils inside the core to learn about the species that survived the asteroid. Lowery, the UT postdoc, dates each core based on the fossils it contains. “The things that survived were the ones with a generalist lifestyle that could do well in any particular niche in the ocean,” he says.

Seismologists, who study earthquakes and the movement of seismic waves, did much of the heavy lifting before the expedition by mapping the crater’s geology. UT senior research scientist Gail Christeson said it was exciting to finally see the sediment she’s been studying since 1994. “We can see craters on the Moon, Venus, and Mars,” she says, “but all you see is the surface. This is the best-preserved impact on Earth, where we can actually see the third dimension in depth.”

Later, an astrobiologist will analyze the fossils’ ancient DNA. “We expect to find evidence of a niche ecosystem, of life that could tolerate a lot of heat and extreme conditions after the impact,” Gulick says. There’s a chance that what scientists learn about how life rebounded after the asteroid could relate to the search for life on other planets—guiding them in what signs to look for underground on Mars, for example.

The core then goes into a freezer to be preserved until September, when the entire research party of 33 scientists will gather at a core repository in Bremen, Germany, to do more thorough analyses. While it will take several years for all the findings to be published, it’s already clear that the Chicxulub expedition will yield major insights. “A lot of PhDs are going to get done on these cores,” Gulick laughs.

But many unknowns remain. “We’re still learning about how impact cratering works fundamentally as a process,” Gulick says. “We don’t know how peak rings are formed. And we don’t know how rocks can appear to flow like a liquid during an impact, so that a crater like this can form within five minutes. That’s why we’re studying what happened at ground zero.”

Exploring one of the Cenotes de Cuzama.

In nearby Mérida, the closest city to the expedition, a museum on Maya culture has a small exhibit on the crater. There’s a cheesy video re-creating the impact, complete with ancient creatures plaintively bleating as the asteroid fills the sky, and a life-sized sculpture of a dinosaur awaiting his fate. A diorama explains cenotes, sinkholes unique to this region that are formed by the collapse of limestone bedrock. The cool, blue water within draws tourists to snorkel and scuba dive and puts Texas’ prettiest swimming holes to shame. If you plot the location of these cenotes on a map, they form a ring that mirrors the shape of the crater—the only sign of the asteroid still visible today.

My favorite part, though, is a timeline that depicts the Earth’s history as though it lasted 100 years instead of 4.5 billion. On that scale, the asteroid didn’t hit until year 98. For all its Armageddon-like qualities, it was only one of five major extinctions the planet has weathered. And while the impact and its effects destroyed millions of species and ended countless lives, we may also have it to thank for our own. “By eliminating the dinosaurs, the impact cleared the way for mammals to evolve,” Lowery says. “So the mass extinction definitely had a role in the fact that we’re here today.”


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