California’s drought and a bark beetle epidemic have caused the largest die-off of Sierra Nevada forests in modern history, raising fears that trees could come crashing down on people or fuel deadly wildfires that could wipe out mountain communities.

Aerial images show vast forests that have turned a rust-color. The epidemic has killed an estimated 40 million trees since 2010 in the central and southern Sierra, and it’s spreading north. Officials who are cutting down and stacking the most dangerous trees in piles across six counties, however, say they are stumped by how to get rid of them all.