A biodiversity researcher has found a huge basalt rock formation in the Taiwan Strait, resembling a city wall and rivaling similar monoliths on land.

The 200 metre-long, 10 metre-high undersea wall, which looks like thousands of pillars packed together, is near the Pescadores archipelago, researcher Jeng Ming-hsiou said today.

Jeng, who is a professor at the state-run Academia Sinica in Taipei, was diving in the area when he saw and filmed the wall, about 40 km west of Taiwan's main island.

"It was completely unexpected," said Jeng. "It's not easy to see these formations underwater."

Basalt walls such as the famed Giant's Causeway on the coast of Ireland and the Wairere Boulders of New Zealand are known to have occured on land but seldom, if ever, found at sea.

The Taiwan Strait formation, which local media have compared to a city wall, likely began with a volcanic eruption as far back as 1,800 years ago, Jeng said.