Two of them are believed still active.

The mud volcanoes, which are formations of sand and rock fragments with very high temperatures, were found during a 16-day mission to survey areas around an abyss about 2,800 metres deep _ the deepest part of the Andaman Sea off Thailand.

"We've never known before there are mud volcanoes under the Thai sea," said oceanographer Anond Snidvongs, director of the Southeast Asia START (System for Analysis Research and Training) regional office. "We cannot say exactly now what our discovery will lead to. We need to gather more information," he said.

The four volcanoes are located about 300km off Phuket island. One of them is shaped like a bell with a height of 100m and a base stretching out around 100km. The other three are smaller.

"Their temperatures are close to 100 Celsius, but nearby sea temperatures are recorded at less than 5C," Mr Anond said. He initially expected two of them might still be active, but added that their eruptions would likely cause no serious impact unlike volcanoes on land because they would just release certain gases, such as methane.

His team discovered the volcanoes by chance as they were studying morphodynamic and slope stability in the Andaman Sea and continental shelves.

The study, which ended on Dec 6, would shed light on the causes of land subsidence under sea.

Mud volcanoes have earlier been located in the Andaman Sea. The one located near Andaman island in India, which is still active, is believed to have links with tsunami waves caused by massive quakes under the sea.

After the Dec 26, 2004, tsunami hit the coastlines in Indonesia and Thailand, this mud volcano has become even more active, scientists say.

However, Mr Anond said it was too soon to link the ones discovered in Thailand with the tsunami. More information was needed to draw a correct conclusion. Mud volcanoes usually signify a deposit of petroleum, like the ones found in the Gulf of Mexico, he added.