Alaska’s Shishaldin volcano began producing long seismic tremors that geologists say could be signs of an impending eruption.

Shishaldin is located on Unimak Island in the Aleutians and is unique among volcanoes in the state.

Rather than containing a lava done or crater, the Alaska Volcano Observatory says the symmetrical, conical volcano has a deep, open vent.

“It’s gas bubbles coming up through the throat or the vent of the volcano. And when they pop, it just kind of throws magma up into the air,” observatory geologist Robert McGimsey told Alaska Public Radio, describing a typical Shishaldin eruption.

The mountain is the most symmetrical and conical volcano in the world because lava from its eruptions glide down the flanks, leaving a smooth layer.

Photo: U.S. Coast Guard