Iceland declared a state of emergency and police have evacuated a town about 40 kilometers (25 miles) southwest of the country’s capital over the looming danger of a volcanic eruption.

Authorities have been on alert after weeks of tremors indicated a magma intrusion is forming underground close to Grindavik on the Reykjanes peninsula.

The fishing town of about 3,600 people sits not far from the iconic Blue Lagoon, Iceland’s biggest tourist attraction, and the Svartsengi power plant owned by HS Orka hf, which provides heat to about 30,000 inhabitants of peninsula.

“There is a likelihood that a magma intrusion has extended beneath Grindavík,” Iceland’s Met Office said in a statement after activity in the town intensified on Friday to near-constant tremors.

Authorities are building fortifications in an attempt to protect the area’s infrastructure, and shut the Blue Lagoon on Friday for at least a week as a precautionary measure.

There have been three eruptions in the area since 2021 after volcanic activity there was dormant for about 800 years. Earlier eruptions were so-called fissure eruptions, with flowing lava.

If a rift were to extend and open in the sea off Grindavik an explosive eruption is possible, which may interrupt air traffic at Reykjavík’s Keflavik Airport, Thor Thordarson, a professor in volcanology at the University of Iceland, said in a telephone interview.

The impact on air traffic is likely to be less dramatic than in 2010, when the Eyjafjallajokull volcano in Iceland’s south released a vast plume of ash that grounded air traffic across Europe for weeks, he said.

Iceland, which has 30 volcanic systems and more than 600 hot springs, is one of the most geologically active places on earth, due to its position on the mid-Atlantic ridge where the North American and Eurasian tectonic plates meet.

Though eruptions aren’t infrequent, residents haven’t experienced an event which threatens inhabited areas of this scale since an eruption in 1973 in the Westman Islands buried part of a town of some 5,000 people under lava.

Whether magma reaches the surface on the Reykjanes peninsula this time is unclear, but “this is by no means over,” Thordarson said. “It is not a question of if but when it erupts.”