Yes, that’s a pretty big plume of smoke you see coming out of Tanzania’s Mount Meru volcano—but contrary to appearances and some reports, it’s not erupting. Something else is happening up there.

NASA’s Earth Observatory cleared up the explanation behind the huge plumes of smoke satellites and aerial observers have been noticing coming from the volcano over the last few days. Many observers concluded, not wholly unreasonably, that they were seeing the first eruption of the volcano in over a century. The actual explanation though is much stranger: It’s a giant wildfire, located all along the tops and sides of the volcano.

How can you tell? Here’s NASA’s explanation of how to tell the difference:

Depending on the event, the difference between a plume from a fire and from an eruption can be subtle. In natural-color satellite imagery, a volcanic plume that contains mostly steam can look very similar to smoke from a fire. However, volcanic plumes tend to become bluer when sulfur dioxide levels rise, and greyer and browner when ash content is high. In this case, Klemetti notes there are several clues that suggest the plume’s origins are not volcanic. There have been no other signs of renewed volcanic activity, such as seismic shaking. Nor have satellites picked up any evidence of sulfur dioxide within the plume. And the plume emanates from the mountain’s northern flank, which is outside the caldera wall where the last few eruptions have occurred.