Much of Hong Kong and its Victoria Harbour, including this rocky islet, lies atop the remnants of an ancient supervolcano with an 11-mile (18 kilometer) wide caldera, Chinese authorities revealed last week.

But even though the volcano has produced some gigantic eruptions, it poses no risk to today's millions of Hong Kong residents. It's been extinct since it last erupted and largely collapsed into the ocean 140 million years ago, Hong Kong's Geotechnical Engineering and Development Department said.

In fact, geologist Denise Tang said in a state-sponsored broadcast, the biggest risk is to tourists trying to visit the rocky islands by boat.

"There are no facilities here so we cannot recommend trying to land," she said.

(Related: "Ancient Supervolcano Rocked Washington State.")

—Richard A. Lovett

Pictures: New Supervolcano Found in Hong Kong

Bristling with hexagonal rock formations, the remains of a giant volcano have been identified in, around, and under the city.