WELLINGTON, New Zealand -- Volcanologists were puzzled Monday about why a lake atop a rumbling volcano on the South Pacific island of Ambae has changed color from blue to bright red.

Mount Manaro, one of four volcanos currently active in the island nation of Vanuatu, has been showing signs of erupting for only the second time in 122 years.

"We are still ... trying to understand this change of color in the lake from blue to red," Geology and Mines Department director Esline Garae told The Associated Press in a telephone interview from the Vanuatu capital, Port Vila.

She said two scientists on Ambae Island were monitoring Lake Vui as well as seismic activity on the 1,500-meter (5,000-foot) Mt. Manaro.

"If the change of color ... comes from new activity in the ground or just chemical change in the lake - these are two things I want to know from those guys before I can say anything" about the danger posed by the volcano, she said.

Mt. Manaro last erupted in November 2005, forcing half the island's 10,000 inhabitants to evacuate their villages but causing no injuries. The eruption before that, in 1884, killed scores of villagers.

Three other volcanos in Vanuatu - Lopevi, Yasur and a two-crater volcano on Ambryn Island called Marum and Benbow - have spewed rocks, ash, smoke and steam over the South Pacific island nation in recent weeks.

However, activity has slowed in recent days, Garae said.

New Zealand volcanologist Brad Scott said Lake Vui's color was "quite a spectacular red," but what had caused it "is the 64-thousand dollar question."

He said water samples from the lake would help determine what was happening in the crater and below it.

The color change "could be a chemical process ... or magmatic gas (from molten volcanic rock) or something else coming into the lake," he said.

The peak on uninhabited Lopevi Island was the most recent to spurt sulfurous ash into the sky, causing havoc on 10 surrounding islands.

Garae said hundreds of inhabitants of nearby Paama Island fled from the falling ash, dense smoke and lava flows from Lopevi, she said.

"It was in a high state of eruption because ash was falling on Ambryn and Paama islands, but then ... ash flows ceased some days ago," she said.

The Yasur volcano on Tanna Island in the south of the archipelago was "of more concern for tourists and tourism agencies," who visit it frequently, she noted.

"A reminder that it is at Level 2 ... forbidding people to go close to the mouth of the volcano" had been posted after it tossed out rocks and some lava.

She said further observations of Yasur would likely be made next week to confirm whether the activity was continuing to decline.

Vanuatu, formerly called the New Hebrides Islands, is comprised of 13 main islands located 2,300 kilometers (1,400 miles) east of northeast Australia.