Solheimajokull glacier is part of the ice cap sealing Katla, a volcano 10 times more powerful than neighboring Eyjafjoell, whose eruption last week clogged Europe's skies with ash and grounded the continent's airlines.

If Katla blows up, the current eruption will resemble "a small rehearsal," Iceland's president, Olafur Grimsson, warned.

The frightening reputation of Katla is mirrored in a savage landscape of black volcanic ridges, blue ice, and snow swept and hardened by powerful winds.

Reaching the glacier requires a four-wheel-drive vehicle, or mountaineering equipment and snowmobiles to go further. During a visit Thursday, a huge raven on a boulder was the sole living creature visible.

The worry for Icelanders is that each time Eyjafjoell has erupted over the last millennium, Katla, named after an Icelandic witch, quickly followed.

Explosive percussions heard every few minutes here testify to lava explosions inside Eyjafjoell, a few kilometers (miles) to the west, and the threat that Katla might be awaiting its cue.

"There have been three Eyjafjoell eruptions and Katla has followed each time," said geophysicist Sigrun Hreinsdottir, at the Earth Sciences Institute. "They are very close."

Some believe the volcanoes are directly linked underground so that magma from one can flow into the other. Hreinsdottir said that what happens inside volcanoes is largely a mystery.

What's known is that Katla has erupted approximately every 80 years since Vikings first settled this island nestled under the Arctic Circle more than a thousand years ago.

The last eruption was in 1918 and "it's the longest pause of Katla on record, which is why we are monitoring it very carefully," Hreinsdottir said.

An eruption from Katla might not necessarily be dangerous. But it has the potential every time to repeat the 1918 scene when a wall of melted glacier water swept down, bearing ice chunks the size of houses, and blanketing southern Iceland in thick ash.

What effect a monster eruption would have on a Europe crippled by the much milder Eyjafjoell can only be imagined. Much would depend on wind direction, the type of ash and height of the ash plume.

Living directly under Katla and the huge Myrdalsjokull glacier means a waiting game equally scary, frustrating and tiresome.

Elias Gudmunsson, manager of the Vikurskali cafe in the seaside village of Vik, says he's fed up with being questioned about the danger.

"Someone from a radio station called me yesterday and asked, 'Are you OK? Are you all still alive?'" he said, laughing humorlessly.

All the same, Katla -- known with the volcano Hekla as Iceland's "angry sisters" -- looms darkly in his life.

"We were brought up on this," Gudmunsson said. "In school we were told stories about Katla and we trained to evacuate."

In case of a major eruption, a towering wave of melted glacier water could rush from the mountain across the narrow, flat coastal strip and, potentially, into Vik.

"I live in a house on the flat, so when Katla goes and the eruption starts we have to move up the hill," Gudmunsson said. "The odds are not very high but it can happen and it has happened."

Iceland's volcanologists watch Katla the way police keep suspected bomb makers under surveillance.

Monitors linked to Global Positioning Satellite (GPS) devices ring the volcano, measuring minute movements in the ground -- whether Katla is shifting up, down or across.

Similar sensors on Eyjafjoell detected significant motion during the current eruption as the mountain heaved and convulsed, says Kristin Vogfjord, a geologist at the Icelandic Meteorological Office.

But there has recently been "none around Katla," Vogfjord said. "There is no seismicity in Katla."

Those reassurances have not calmed a torrent of speculation, much to the annoyance of Iceland's growing tourist industry.

When the president referred in a BBC interview to the current eruption being a rehearsal for Katla, many here were furious.

Even Finance Minister Steingrimur Sigfusson joined critics, saying "we cannot give out the message that chaos or a frightening situation reigns."

In her farmhouse at the foot of the mountains leading to Katla, Andrina Erlingsdottir said she was sick and tired of waiting for something that seems inevitable.

"We are waiting for Katla. I hope it will come soon," she laughed. "But I think we are the only ones hoping for that."

Her family runs tours to the Myrdalsjokull glacier on top of Katla and business is hurting.

She says her family would only have 15 minutes to evacuate if Katla's eruption sent a flash flood down the valley near her house.

After 16 years of glacier tours, Erlingsdottir has mixed feelings about this hidden menace.

"It's really peaceful up there. You don't even feel earthquakes when you're on the glacier," she said.

But the legend about the mountain hiding Katla the witch is never forgotten.

"She gets angry," Erlingsdottir said. "You better not bother her."

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