New satellite imagery of the 5,676-foot peak, located about 940 miles southwest of Anchorage in Alaska's remote Aleutian islands, shows a new, bulging lava dome forming in the volcano's summit crater, suggesting a buildup of gas pressure.
Even though the volcano is in an uninhabited area, an eruption could still cause a big mess. According to CNN, some 90 percent of air freight from Asia to Europe and North America passes over Alaska, along with hundreds of commercial flights.
Officials say that intermittent and sudden explosions could happen at any time, generating ash clouds up to 20,000 feet above sea level.
The Alaska Volcano Observatory has elevated the Aviation Color Code from Yellow to Orange, indicating that Cleveland Volcano is exhibiting "heightened or escalating unrest with increased potential of eruption." Orange is the second highest warning level in the AVO's four-color alert system.
The observatory says that as of Monday, the lava dome was approximately 130 feet in diameter.
The volcano erupted twice recently, on Christmas and again on December 29, sending an ash plume 15,000 feet high.
But that was just a rumble compared to the eruption in 2001, which saw ash clouds as high as 39,000 feet.