At the bottom of a lagoon at a nature reserve in England, a secret lay dormant for millenniums, hidden by mud, water and ice.

Only recently had the fossilized remains been found, a throwback to when dinosaurs ruled the Earth and gigantic marine reptiles, colloquially referred to as “sea dragons,” marauded the oceans.

This was not just any old find: The remains of the sea creature, an ichthyosaur, were the largest ever discovered of that type in Britain, those involved in a now-complete excavation project announced on Monday. They said it was also one of the largest and most complete skeletons of an ichthyosaur (pronounced IK-thee-uh-sor) found anywhere in the world.

The skeleton is from the early Jurassic Period about 180 million years ago and measures about 10 meters (more than 30 feet), they said. It might never have been unearthed if the lagoon hadn’t been drained as part of a landscaping project.

The fossil was found in 2021 at the Rutland Water Nature Reserve in England’s East Midlands, a landlocked reservoir about 160 kilometers (100 miles) north of London that is known for attracting water fowl and other birds.

Joe Davis, a conservation team leader at Leicestershire and Rutland Wildlife Trust, said on Monday that he first encountered the fossil last February as he plodded through the mud in his waders with a colleague.

“We sort of looked at it and scratched our heads,” Mr. Davis recounted in an interview. “I realized it might be something from the dinosaur era. We could see these ridges and bumps. That’s when alarm bells started to ring.”

Mr. Davis, 48, took photographs of the fossil and contacted the Rutland County Council, which connected him with a geology curator at the University of Leicester, who referred him to Dean R. Lomax, a paleontologist specializing in the study of ichthyosaurs.

“I immediately recognized them as ichthyosaur vertebrae,” Dr. Lomax, the head of the excavation project, said on Monday. “He had found this so serendipitously.”

Ichthyosaurs, fish-shaped marine reptiles that resemble whales and dolphins, first appeared about 250 million years ago, according to Dr. Lomax, who said that they were apex predators that likely feasted on other ichthyosaurs, fish, another reptile known as a plesiosaur and ammonites, a kind of mollusk. They disappeared about 90 million years ago and overlapped with dinosaurs, he said.

“They had these big eyes, big teeth,” he said. “A lot of people tend to go back to the old days and call them sea dragons.”

From the photos, Dr. Lomax said, he could not tell whether the specimen was an entire skeleton or just fragments like many of those that had been discovered over the centuries in England. He needed to see it for himself.

So about two weeks later, he said, he led a one-day, mini-excavation to the nature reserve with four paleontologists.

“We were all blown away by it,” said Dr. Lomax, 32, a visiting scientist at the University of Manchester.

But the conditions at the nature reserve didn’t lend themselves to a full-scale excavation. The lagoon was frozen and would eventually need to be refilled with water so as not to disturb the natural habitat, according to Dr. Lomax, who said that the paleontologists covered the skeleton with plastic sheets and mud until they could return.

“As an expert, I was itching to get there and excavate it,” he said. “We had a bunch of migratory birds there as well. We had to wait for them to leave.”

In August, a team of experts that included Dr. Lomax returned to the site for several weeks to excavate the skeleton, taking daily tests for the coronavirus and signing nondisclosure agreements saying that they would keep the discovery a secret.

“The skull weighs over a ton,” said Mr. Davis, who made the initial discovery and whose son kidded that the skeleton was from the “Joe-rassic” era.

To protect the skeleton as it was hoisted from the ground, it was wrapped in plaster, which Mr. Davis and Dr. Lomax likened to molding a cast for broken bones. Dr. Lomax lay down on the ground next to the excavated skeleton to show its size.

Mr. Davis said it was fortunate that the skeleton was not damaged when the lagoon was initially excavated 12 years earlier.

“They must have been inches away when they originally constructed the lagoon,” he said.

It could take 18 to 24 months to preserve the skeleton and remove the rock from the bones, according to Dr. Lomax, who said those involved in the project hoped to display the specimen in the Rutland area. Skeletal remains of ichthyosaurs have more typically been found along the Jurassic Coast in southern England, he said.

After the skeleton was removed from the ground, it was taken by trucks to the laboratory of Nigel Larkin, the project’s co-leader, which Dr. Lomax said was about a 2.5-hour drive from Rutland. The main body section was too large to fit inside a truck, so it was loaded onto a trailer — not that other drivers would have noticed. It was wrapped.

Said Dr. Lomax, “It would have given people a good scare.”