Volcanoes in the Nusa Tenggara chain of islands are rumbling awake, prompting authorities to raise the alert level on three of them over the past six days.

Sutopo Purwo Nugroho, a spokesman for the National Disaster Mitigation Agency (BNPB), said on Thursday that the three volcanoes had been placed on the second-highest alert status. They include the infamous Mount Tambora, on Sumbawa Island, whose 1815 eruption was one of the worst in Indonesia’s history.

A total of 16 volcanoes across the country are now at the same status.

“We haven’t confirmed what’s causing the increased volcanic activity in Nusa Tenggara because it will require a long and comprehensive study,” Sutopo said.

The status of Tambora, located between Bima and Dompu districts in West Nusa Tenggara, was raised at 11 a.m. on Tuesday. It first started showing signs of increased activity in April. In early August its Doro Api Toi crater spewed thick white smoke 20 meters into the sky.

The volcano’s April 10, 1815, eruption claimed the lives of at least 90,000 people, including those who died in the aftermath of the eruption from famine and disease. The eruption is estimated to have had a Volcanic Explosivity Index ranking of 7 — a supercolossal event that ejected immense amounts of volcanic dust into the upper atmosphere, significantly impacting the global climate for many years afterward.

On Wednesday, the status of Mount Lewotobi-Perempuan, on Flores Island in East Nusa Tenggara, was raised after volcanologists registered an average of 24 earthquakes a day, far higher than its usual five a day.

Mount Anak Ranakah, in Manggarai district in East Nusa Tenggara, had its status raised on Friday. Volcanologists noted an increase in the number of deep and shallow volcanic earthquakes since June. When it last erupted on Jan. 11, 1988, the mountain spewed smoke as high as 8,000 meters into the sky.

But Sutopo said there was not yet a need to order evacuations. “We just want the local governments and communities to increase their awareness,” he said.

“We also strongly suggest that climbing activities be stopped,” he added.

Aside from the 16 volcanoes on the second-highest alert status, another 21 volcanoes in the archipelago are on heightened alert.