Geotech engineer realised it all pointed to a supervolcano

The man behind the discovery of Hong Kong's supervolcano remembers the moment he realised he was on to something big - the explanation for the city's unique landscape.

"The 'Ah-ha!' moment for me was realising that everything pointed to one source - a supervolcano, the one system that could have preserved all the unique geological features of Hong Kong," said Dr Roderick Sewell, who has worked as a geologist in the Civil Engineering and Development Department for more than 22 years.

Sewell recalled suddenly comprehending: "We're seeing something much bigger than what we thought."

In the four years since his epiphany, he and his colleagues in the Geotechnical Engineering Office (GEO) have thrown themselves into long periods of survey work and careful calculations that resulted in the unveiling of the High Island Supervolcano yesterday - 140 million years after it erupted.

In the mid-1990s, GEO geologists' work on precise dating of rocks and geological mapping revealed that 80 per cent of Hong Kong's landscape could be attributed to volcanic eruptions. They put it down to a series of eruptions, however, rather than one supersized event.

As their calculations took form, Denise Tang Lai-kwan, a fellow geotechnical engineer from GEO, was shocked by the scale of what was unfolding.