The haphazard history that gave Hong Kong some of the most valuable real estate in the world has just got more random. Geologists announced yesterday that we are only here thanks to an extraordinary volcanic eruption. It was one of only about 50 supervolcanoes to have erupted since man walked the planet, none of which have occurred in recorded history. And now, 140 million years after the event, its existence has been unveiled by geologists who realised what they were looking at during the course of their daily survey work. "It is correct to say that the main city sits on a very old volcano," Denise Tang Lai-kwan, Civil Engineering and Development Department geotechnical engineer said. "Though most of the remnants are submerged, the granite in Kowloon and the rock pillars in Ninepin Islands can be traced back to this one source." Her colleague, Dr Roderick Sewell, added: "There are only around 50 supervolcanoes in the history of mankind. We suspect more will be added to the list following our discovery." Geologists have mapped out its entire anatomy running from eastern Sai Kung to Kowloon and Hong Kong Island itself, formed by the lava and ash left behind. It has been named the High Island Supervolcano after the Sai Kung island, which marks the volcano's top edge. The 18-kilometre-wide caldera (a kind of crater) of the supervolcano was formed as high pressure and heat at the earth's core caused the surface to collapse. The deepest part of the supervolcano is in the middle of the city, in Kowloon and Hong Kong Island, where leftover pockets of magma are marked out in the form of granite.