The High Island Supervolcano's existence was discovered as a by-product of the government's surveying work in geological mapping, geochemistry, geophysics and dating of rocks conducted by the Hong Kong Geological Survey.

Staff had a hazy idea in 2008 that a supervolcano lay underneath the geoparks in Sai Kung but it took a number of years to confirm the discovery and then publish the findings in a US geophysical journal.

"This discovery was the result of years of very detailed, meticulous surveying work and very careful dating of the rocks," said geological survey head Roderick Sewell.

"It took a very long time to piece everything together into a story.

"It's a long road to discovery but when you suddenly get the `aha! moment,' it makes it all worthwhile."

The progressively deeper parts of the supervolcano extend from Sai Kung toward Kowloon and Hong Kong Island, revealing a complex plumbing system of how magma used to feed the volcano.

People who are interested to see the remnants of the volcano may visit the geoparks on Ninepin Island, Sharp Island and Tung Ping Chau.

The last time it erupted was at least 140 million years ago in the Mesozoic era when dinosaurs were once dominant.

Sewell, who assured it will never erupt again, thinks the discovery will attract people to the geoparks, and stimulate others to learn more about geology.

There had been no supervolcanic eruptions in recorded history and the last to erupt was in New Zealand at least 26,000 years ago, said geological engineer and geologist Denise Tang Lai-kwan.