I can hear the throaty rumble, like a Harley in need of a tune-up, even before I round the dunes. Scrambling to the top—and staying safely behind my red-coated guide—I can see two massive brown blobs of rippling blubber facing off on the beach below. Looking a bit like slugs on speed, they arch up to their full heights, bulbous noses swinging.

Suddenly the fight begins. Lunging faster than their bulk should allow, the two testosterone-crazed males tear at each other with sharp canine teeth. The thick fat on their chests acts as a sort of armor, but the results of this battle for status still aren't pretty. Blood quickly coats their broad chests, coloring the foaming seawater around the massive beasts pink.

Less than a minute and it's all over. The winner hurries back onto the beach, to his spot amidst the dozens of female seals in his harem. The loser, vanquished, lays in the crashing surf for a while, then swims a short way down the beach.

The battling beasts are elephant seals, one of the California coast's most curious spectacles. Every winter, thousands of them haul up on beaches from Baja to just north of the San Francisco Bay. You can't quite call them majestic, or even particularly good looking. From a distance, they resemble tubes of Jell-O rippling across the beach. But for four months a year, people can get nearly within spitting distance of the largest seal in the sea.

On a blustery morning, I headed to Año Nuevo State Reserve to see the seals for myself. The beach at the California state park is the most important seal rookery on the West Coast. Four different seal species mate, rest, give birth and feed on the beaches and in the water of this rocky Pacific point. Located an hour and a half's drive south of San Francisco, the park is also home to sea otters, America's rarest snake and an endangered cousin of the penguin called the marbled merlet.

But the elephant seals are the unlikely stars of this circus. Named for the large, dangling noses of the males (for seals, it seems, size does matter, and the bigger the better when it comes to attracting mates) the seals spend most of their lives at sea, coming ashore only to mate, give birth and molt. Between December 15 and the end of March, the beaches are filled with seals the size of SUVs mating and giving birth.

Best of all, the park is open to the public for guided tours all through mating season. Spaces on the two-hour walking tours fill up months in advance; rangers and volunteer docents guide almost 50,000 people through the dunes in three and a half months. "Here people are able to see the whole life cycle in the wild," says Año Nuevo State Park Ranger Frank Balthis. "Watching the season unfold is like reading a good novel."

Just a century ago, it looked like the book on elephant seals was closed. In the 1800s, they were killed in the thousands for their blubber, like vulnerable, beach-going whales. But a small number of seals (less than 100, experts believe) eluded hunters and survived on remote islands off the coast of Baja California. When the struggling population was found in 1892, seven of them were killed and sent to the Smithsonian. Despite the depredations of museum collectors and poachers, the colony held on.

Things took a turn for the better in the 1920s, when the Mexican and American governments recognized them as a protected species. Since then, they've made a remarkable comeback. From that single Baja population, there are now more than 150,000 swimming in the Pacific—and flopping heavily ashore each winter from Baja to the northern California coast. Today, Año Nuevo is the largest mainland elephant seal rookery on the California coast.

Breeding season is a singular spectacle. First to arrive are the males, whose goal is to carve out a patch of sand they can defend from other males. As female seals begin to arrive, the largest and most aggressive males gather them into harems. At the height of the breeding season, more than 2,400 females pack the beaches under the watchful black eyes of massive alpha males.

Females land on the beach heavily pregnant from the previous breeding season and eager to find a strong male to protect them. They give birth almost immediately to a black-coated pup weighing about 75 pounds. The far larger alpha males, on the other hand, spend the breeding season fighting to guard their females. As I stand watching the action on the beach, a small male sneaks into the closest harem and climbs on top of a squealing female. He jiggles away as soon as the alpha male begins to lumber over.

The move isn't unusual: dominant males are constantly tested. Older males' chests are covered in thick carpets of scar tissue from countless tussles. Only one in 20 males are big and aggressive enough to have their own harem.

To head off the rest, the alphas must be on constant watch. Breeding season is a brutal endurance contest—the males spend up to four months straight on the beach, not eating or drinking. Full-size males can be 16 feet long, with the biggest weighing in at 4,500 pounds, roughly the size of a Chevy Blazer SUV; they'll lose a third of that before heading out to sea again.

The most successful males will mate with 50 females during the winter months. "Imagine not eating for four months and fighting off competitors and breeding with females," says Samantha Simmons, a marine biologist at the University of California at Santa Cruz and an expert on elephant seals. "It's hard."

Researchers have been visiting Año Nuevo for decades to watch the seals and learn about their life cycle. But for decades, the rest of the elephant seals's story remained a mystery. What did they do in the eight long months they spent at sea? Where did they go, and what did they feed on to pack on all those pounds?

The answers began to come in the 1980s, when University of California at Santa Cruz biologists began gluing satellite tracking tags to the seals' fur with marine epoxy. What their data revealed was stunning: elephant seals can spend up to two hours underwater, longer than sperm whales, and dive to depths of almost a mile looking for food. While at sea, they spend just a few minutes at a time on the surface. "They're the consummate divers," says Simmons. "We should almost call them surfacers rather than divers—the vast majority of their life is spent under the surface of the ocean at depths we just don't understand."

Andrew Curry is a writer based in Berlin, Germany.