0350 GMT January 24, 2021
The pinkish-flowered plant, known as grass poly, was found growing on the banks of an old farmland pond in Norfolk, England, BBC reported.
The mystery species "came back from the dead" after seeds submerged in the mud were disturbed during work to restore the pond.
And scientists say conservation efforts could lead to the return of other long-forgotten botanical gems.
Carl Sayer, a professor at University College London (UCL), stumbled on the plant when he went to survey the pond at Heydon shortly after the first national lockdown ended.
Having never seen anything like it before, he quickly snapped a picture, which he sent to local botanist Dr. Jo Parmenter.
She identified it as grass poly, one of the rarest plants in the UK.
"It's really quite beautiful," saidProfessor Sayer. "We only found a handful of these plants in the pond but we're hoping to cultivate this population and keep it going and expand it now we know it's there."
Parmenter was thrilled to see the photo of the plant. "I never ever expected to see it in Norfolk; it was quite extraordinary," she said. "I saw a photo and straight away I thought, I know what you are."
The last confirmed record for grass poly (Lythrum hyssopifolia) in Norfolk dates to more than a century ago.
Elsewhere in the UK, the plant is found in a few isolated populations growing around lakes and on muddy open ground.
At Heydon, the seeds of the plant remained buried in the mud, like a "time capsule". When willows were pulled out to restore the pond, this disturbed the soil and let in light, allowing the seeds to germinate.
"There's no oxygen, it's very dark, and it's perfect for preserving seeds," said Sayer, who is part of UCL's Pond Restoration Research Group.
The discovery shows plants believed extinct can be brought "back to life" with good conservation, he added.