JAKARTA - SCIENTISTS scouring the ocean floor to study the nature of tsunamis discovered a massive underwater volcano off Indonesia's western coast.

The 4,600m high mountain spans 50km at its base, Yusuf Surachman Djajadihardja, a marine geologist with the government's Agency for Assessment and Application Technology, said on Friday.

Its discovery was 'completely unexpected,' he said.

It was not immediately clear if the volcano is active, but he said if it were and it erupted, it would be 'very, very dangerous.'

An international team of scientists discovered the volcano 330km west of Sumatra island while carrying out a survey of the Indian Ocean floor earlier this month, Dr Djajadihardja said. The tip of the volcano is 1,380m below the water's surface.

The goal of their research was to better understand the cause of disasters like the 2004 Asian tsunami that killed 230,000 people in a dozen nations, more than half of them in Indonesia, which was closest to the epicentre.

Indonesia straddles a chain of fault lines and volcanoes known as the Pacific 'Ring of Fire' and is prone to seismic activity. -- AP