Help the local news anchor complete the breaking news report about a clumsy criminal.

The bank robber ran _____ the dark forest, tripped over a tree root, and accidentally dropped all the stolen cash!

The correct answer is through.

We use through when moving inside a three-dimensional space or volume (like a forest, a tunnel, or a crowd). We use across for moving from one side to another on a flat two-dimensional surface (like a field or a street).