Hawaii island has been under an advisory, designated by the current volcano alert system, for the past two years.

Mauna Loa, the largest and most active shield volcano in the world, continues to show increased signs of restlessness beneath its surface.

Its last eruption was back in 1984, but scientists say they’re certain it’ll erupt again. It’s just a matter of when.

So what’s being done to keep a closer eye on Mauna Loa?

The U.S. Geological Survey’s Hawaiian Volcano Observatory is installing two sensors this month. One will be put on the southwest rift zone to monitor different gas emissions.

Tamar Elias, HVO gas geochemist said, “as things change, subsurface changes in the volcano, we might expect to see changes in the types of gases that are emitted and so now we will have a way to monitor that.”

Another will monitor earthquakes high on the east flank.

“It’s also pretty close to the summit of Mauna Loa, so it’s only about six kilometers away from the summit caldera, so it will be our closest station to the summit,” said Brian Shiro, HVO seismic network manager and geophysicist.

Shiro says most of Mauna Loa’s eruptions started at its summit.

“After a brief eruption at the summit, then it propagates down one of the two rift zones to the northeast or southwest and opens up in a fissure at one of those places for a more sustained eruption,” he said. “Once that fissure opens up, in some cases Mauna Loa eruptions have reached the ocean very quickly in as little as three hours from that point, so they can be fast-flowing eruptions.”

Mauna Loa has erupted 33 times since 1843, an average of once every five years, according to the USGS.

One of the most obvious signs before an eruption is an increase in earthquakes, similar to what we’ve seen in the last two years.

“Prior to the last eruption of Mauna Loa in 1984, there was an increase in seismic activity, numbers of earthquakes, which is quite similar to what we’re seeing today,” said Shiro. “Since 2015, earthquakes have increased at Mauna Loa and the mountain has began expanding a little bit, which we can interpret as magma movement underground.”

Elias said, “Eventually this volcano will erupt and it’s important for people on the island to stay in touch with what’s going on with the volcanoes in their backyard.”

The 1984 eruption of Mauna Loa threatened Hilo, but Shiro says another eruption could threaten any number of communities.

“There are communities that are in the path of potential lava flows of Mauna Loa on many places on the island, but not one of them is more at risk than any other because we don’t know where the eruption would actually start,” Shiro said. “There were eruptions on the southwest rift that threatened where Oceanview is today and places like Captain Cook, Pahala, Naalehu, any areas down there depending on where the fissure would open up potentially could be threatened.”

Although Mauna Loa hasn’t erupted since 1984, Shiro says it’s important to stay informed.

“It hasn’t erupted in about a generation now, but it will erupt again. We need to be ready for that,” Shiro said. “There’s no sign that an eruption is imminent. It could happen tomorrow. It could happen in another 10 years. We don’t know. But HVO is monitoring it all the time and we’ll be on the lookout for it to spring to action when and if it does erupt.”Click here for regular updates on Mauna Loa and Kilauea from the Hawaiian Volcano Observatory.