In the early morning hours, just as a scientists were set to wrap up a three-week survey of coral reefs 200 miles west of Ireland, something pinged the underwater drone.

Shark.

Indeed, not just one shark, but a swarming school of blackmouth catsharks lit up the sensors of the remotely operated vehicle. And those sharks would soon reveal a forest of coral skeletons studded with thousands of egg cases.

The find is being hailed as a "eureka" moment — the biggest shark nursery ever found in Irish waters.

"It was incredible, real David Attenborough stuff," David O'Sullivan, chief scientist for the SeaRover survey told the Guardian. "This is a major biological find and a story of this magnitude would have been on 'Blue Planet' if they'd known about it. Very, very little is known on a global scale about deep-sea shark nurseries."

The eggs, often called "mermaid's purses," were sown amid corals around 2,500 feet below the surface. That's where researchers from Ireland's Marine Institute had been been directing the SeaRover over the summer.

They shared their findings at the INFOMAR Seabed Mapping Seminar this week.

"This discovery shows the significance of documenting sensitive marine habitats," O'Sullivan explained at the seminar. "And [it] will give us a better understanding of the biology of these beautiful animals and their ecosystem function in Ireland's Biologically Sensitive Area."

Blackmouth catchark swimming in front of a drone camera. A school of catsharks tipped off scientists to a vast nursery amid deep-sea coral. (Photo: Marine Institute)

The area was established by an EU Commission in 2003, setting out strict rules for commercial fishing in an effort to protect its rich marine life. The Biologically Sensitive Area (BSA) is already known as a cradle for herring, cod and haddock, as well as sea fans, sponges, worms and starfish.

Sharks, however, are famously furtive when it comes to their nurseries. Most recently, a major great white nursery was uncovered — also by a drone — in the unlikely waters between New Jersey and Long Island, New York.

With Irish territorial waters 10 times bigger than the island itself, those cold depths represent a tantalizing frontier for scientific exploration. It's not a surprise then that marine biologists are eager to dive back in. At least, remotely.

"There has to be more shark nurseries," O'Sullivan told the Guardian. "The sharks have to be congregating in some areas, but very little is known about the biology and ecology of many shark species."

Underwater drone spies 'incredible' shark nursery off the coast of Ireland
A drone has found a vast catshark nursery on the seafloor west of Ireland, say scientists with the SeaRover survey.