Since July, scientists have been watching Cleveland Volcano for signs of an eruption. The potential for an explosive event has only increased in the last month.

A lava dome has now reached the lip of the volcano’s crater, and if it keeps growing, it could overflow and create an ash cloud. The volcano is located on unpopulated Chuginadak Island, so an eruption wouldn’t create any immediate risk for human life. But Cleveland does lie under a major flight path, and air travel could be affected if the volcano does blow. Ash fall could also impact commercial fishing by damaging boats, and it could also be harmful to people with respiratory problems.

The Alaska Volcano Observatory was last able to get a satellite view of the volcano on Saturday. Because there is no seismic equipment on the volcano, scientists are unable to monitor it in real-time. However, they are able to detect lightning there within minutes, which is usually a sign of a major ash cloud.

The last time Cleveland produced a significant eruption was in 2001, when it created ash clouds that went up nearly 40,000 feet in the air. It also released minor ash emissions in 2009.

Cleveland lies 115 miles from Unalaska. The closest village to the volcano is Nikolski at a distance of 45 miles.