"I rang up the county council and I said I think I've found a dinosaur," explained Joe Davis, who works at Rutland Nature Reserve near Leicester.
During landscaping work at the reserve's reservoir in February 2021, he'd spotted something unusual poking out of the mud.
And it was the largest of its type ever discovered in the UK.
"I looked down at what seemed like stones or ridges in the mud and I said this looks a bit organic, a bit different," Mr Davis told BBC News. "Then we saw something that looked almost like a jawbone."
The council said to Mr Davis: "We don't have a dinosaur department at Rutland County Council so we're going to have to get someone to call you back." A team of paleontologists were brought in for a closer look.
They concluded it was an ichthyosaurs - they were warm-blooded, air-breathing sea predators not unlike dolphins, and could grow up to 25 metres long. They lived between 250 million and 90 million years ago.
Dr Dean Lomax, a palaeontologist from Manchester University, was brought in to lead the excavation effort. He called the discovery "truly unprecedented" and - due to its size and completeness - "one of the greatest finds in British palaeontological history".
Rutland is more than thirty miles from the coast, but 200 million years ago higher sea levels meant it was covered by a shallow ocean.
When water levels at the Rutland reservoir were lowered again in the late summer of 2021, a team of palaeontologists came in to excavate the remains. Special attention was paid to the removal of the huge skull.
A large block of clay containing the ichthyosaur's head was carefully dug out before being covered in plaster and placed on wooden splints.
The block, weighing almost a tonne, was raised out of the mud and will now be examined further.
"It's not often you are responsible for safely lifting a very important but very fragile fossil weighing that much," said Nigel Larkin, palaeontological conservator and Visiting Research Fellow at Reading University. "It is a responsibility, but I love a challenge."
Anglia Water, which manages the Rutland reservoir, is now looking for funding to enable the ichthyosaur to stay in the area and be enjoyed by the general public.
"A lot of people thought I was pulling their leg when I told them I'd found a large marine reptile at work," Mr Davis said. "I think a lot of people won't believe it until the TV programme goes out."
That TV programme is on Tuesday 11 January at 8pm on BBC Two. "Digging for Britain" will then be available on the BBC iPlayer.