This story appears in the July 2019 issue of National Geographic magazine.

Thetis Bay, near the very tip of Tierra del Fuego in Argentina, is about as far south as one can go in the Americas.

Few people ever do. “This is but a bad place for Shipping,” Captain James Cook wrote in his journal in 1768, cautioning future visitors to keep clear of the seaweed. But the bay does provide some shelter from the region’s notoriously rough seas and battering winds. On a chilly, overcast day in February 2018, we launched a Zodiac craft from our ship, the Hanse Explorer, and maneuvered it through Thetis toward the shore, careful to avoid the thick blankets of kelp and the sandbanks emerging at low tide.

I was there leading a National Geographic Pristine Seas expedition, in collaboration with the Argentine government, the regional government of Tierra del Fuego, and the Forum for the Conservation of the Patagonian Sea. With me was my old friend and colleague Claudio Campagna, who co-founded the forum in 2004 and has dedicated his life to studying and protecting the marine mammals of Argentina. Our goal was to gather scientific information and produce a film to lay the groundwork for a new protected marine reserve in Argentina’s waters.

View Images Thousands of juvenile false southern king crabs (Paralomis granulosa) swarm a giant kelp forest near Cape Horn, Chile. The species is heavily fished in the region, and such large aggregations are extremely rare.

Creating such reserves—national parks of the sea—is my life’s work. Over the past decade, our Pristine Seas team has partnered with local allies to help governments protect more than two million square miles of ocean from fishing and other threats. Our expeditions have taken us diving all over the world, from coral reef islands in the vast Pacific to the frozen archipelagoes of the Arctic.

View Images NGM MAPS

The expedition to the tip of Tierra del Fuego was especially important to me—not just for what we might be able to achieve but also because of a personal connection to the place. Back in 1973 Paul Dayton, my friend and scientific mentor, conducted groundbreaking research here. Braving polar winds, hail, and snow, and wearing only old-fashioned wet suits—as opposed to our modern dry suits—Paul and his buddies dived around Thetis Bay and Isla de los Estados (Staten Island) just to the east. They measured and counted giant kelps and the invertebrates living under the kelp forest canopies fringing the shores. Nobody had studied these underwater habitats, and part of our mission was to redo Paul’s surveys. I’ve seen firsthand the dramatic changes in other parts of our oceans caused by overfishing and climate change, the most conspicuous being the bleaching and death of coral reefs and the shrinking of Arctic sea ice during summer. What were we going to find here beneath the surface, 45 years after Paul’s visit?

Claudio and I stepped on the beach and immediately realized we were walking on a mass grave. Old sea lion bones crunched underfoot with every step—the legacy of hunters in the first half of the 20th century. Some skulls had holes made by metal picks. There were jaws and teeth from huge old males and little juveniles. Sea lions and fur seals had been taken indiscriminately, mainly for pelts and for blubber to boil for oil.

View Images Southern sea lions (Otaria flavescens) congregate in Francisco Coloane Marine Park in the Chilean fjords. The park also provides refuge for humpback whales to rest and feed as they migrate annually between the Colombian coast and the waters of Patagonia.

By Paul Dayton’s time, the Argentine government had protected these species by law, but they have yet to recover. According to researchers, local sea lion numbers are a fifth of what they were more than 70 years ago, possibly because of the dramatic decline of reproductive females and the vast footprint of industrial fishing.

“In the past, people killed them directly,” Claudio said. “Now we’re depriving them of their food too.” Three days before our visit to Thetis Bay we’d seen a 360-foot-long supertrawler at the port of Ushuaia. Its nets were big enough to hold a dozen Boeing 747s. Such bottom trawlers and long-liners operate at the edge of the continental shelf of Tierra del Fuego, where the deep basin begins.

Nearer to shore, the weather is so brutal most of the year that few go through the effort to dive at Thetis Bay and Isla de los Estados, but having arrived in relative calm, we were able to dive around the island for two weeks.

The cold, nutrient-rich waters feed giant kelp forests that harbor one of the most magnificent marine ecosystems on the planet. Pillars reach from as deep as 150 feet to the surface, sometimes adding a foot and a half in a day. Giant kelps continue to grow once they reach the surface, creating a canopy through which sunlight filters as if through the stained glass of a cathedral.

View Images Shooting out of the surf, a southern sea lion bull nabs a southern rockhopper penguin (Eudyptes chrysocome) at Isla de los Estados. Southern rockhoppers venture offshore to catch fish, swimming together by the hundreds. There is safety in numbers—for most.

Paul had graciously scanned and copied his handwritten notebooks for us; the pages were filled with detailed natural history observations from 1973. We carried them like treasure. Giant kelp forests all look the same from the surface, but underwater it’s a different story.

Paul had found that every little bay had its peculiarities, sort of an ecological personality. In one bay the kelps were covered only by one or two species of clams; in another, by little soft corals; and in a third, by baby sea cucumbers with finely branched plumes for capturing food particles in the rich seawater.

To our astonishment, the kelps in each bay still harbored the same species. The oceanographic conditions appeared to have remained similar here for the past half century: Climate change had made no permanent mark yet. This seemed a wonderful gift, and I felt a burst of joy.

We were amazed as well by the abundance of life. Every square inch of the bottom was occupied by a living organism: white and yellow sponges, pink encrusting algae, lollipop-like sea squirts. Giant kelps bent to the seafloor from the weight of the mussels growing on them. Blue starfish gorged on the mussels, along with snails and hermit crabs. A year before, on the Chilean side of this ocean ecosystem not far from Cape Horn, we’d stumbled upon a massive aggregation of another crab species, called false southern king crabs. Two layers of them covered the bottom while many more climbed the giant kelps and parachuted down on their fellow crustaceans—and on our heads. It was as if we were part of a Japanese science fiction film. Crabzilla!

View Images A petrel flies over a colony of southern rockhopper penguins at Isla de los Estados. More than 10 percent of the world’s population of this vulnerable penguin species lives here.

One day we took a break from the diving and ventured to the basin beyond the edge of the continental shelf. The Yaganes Basin is the heart of a massive, connected ocean ecosystem that ranges across the southern tip of Chile and Argentina to Antarctica, in a convergence of Pacific, Atlantic, and Antarctic waters. Our engineer, Brad Henning, had brought along several National Geographic Dropcams, glass (borosilicate) spheres enclosing cinema-grade cameras and lights. They have a weight system that carries the camera to the bottom, then returns it to the surface hours later—perhaps with a trove of never seen before footage of the seafloor.

The Dropcams did not disappoint. When Brad showed us a selection of video clips the cameras had captured, our jaws dropped. Toothfish, hake, and other deep-sea fishes flocked to the bait Brad had attached to the Dropcam. At one point a fat red squid approached the camera, then vanished in an explosion of ink. Many of these species are overexploited in the basin. To see them still there means their populations can rebound, if humans let them.

View Images A large southern red octopus (Enteroctopus megalocyathus) rests on a bed of sea urchins at Isla de los Estados. Sea urchins are voracious kelp feeders, capable of denuding entire forests—but here the forests still thrive. Octopuses eat fish and crabs, dead and alive.

After the expedition we changed from wet suit to business suit to lobby Argentine government officials for ocean protection along with our partners at the Forum for Conservation of the Patagonian Sea and Tompkins Conservation. Alex Muñoz, Pristine Seas director for Latin America, presented the results of our expedition to the government, in support of a plan to create the Yaganes marine park. We also premiered our documentary film from the expedition in Buenos Aires, bringing the marine wonders of Yaganes and Tierra del Fuego to Argentine leaders and citizens.

In December the Argentine Parliament convened an extraordinary session to consider the proposal. We were all nervous. We knew that the National Park Administration and some key leaders in government supported protection of the area. But under Argentine law, the bill to authorize the park had to be approved by the House of Representatives and the Senate.

View Images Krill swarm inside a giant kelp forest at Islas Diego Ramírez, 60 nautical miles southwest of Cape Horn. Krill form the base of an enormous food web, supporting fish, penguins, sea lions, fur seals, whales, and—atop the web—orcas, among the ocean’s most formidable predators.

After some tense negotiations, the House voted on December 5. I was astonished. The bill passed on a vote of 196 to zero—as resounding an affirmation for conservation as I’ve ever witnessed in any country. The Senate gave it the final blessing on December 12. Chile had already designated its own fully protected marine park south of Cape Horn a year earlier. Forty years ago Chile and Argentina had come to the brink of war over disputed territorial rights south of Tierra del Fuego. Now the presidents of the two countries would like to declare the area a marine peace park—possibly the largest contiguous transboundary protected oceanic area.

“Today is a day of joy for all Argentines,” Claudio told me on the phone, after the park bill was signed into law. But the joy isn’t only for Argentines. Having had the privilege of exploring and documenting these waters, I feel that the ocean has won a little bit back against our relentless quest to empty it of life. Thanks to the leadership of two governments, the integrity of the great ecosystem of the sea at the end of the world will be maintained for years to come.

National Geographic Explorer-in-Residence Enric Sala is the founding director of Pristine Seas, which—with government leaders, NGOs, and local communities—has helped protect more than two million square miles where ocean life can thrive.