A Curious Finding (Terra Quest 1-7)

Synopsis
Dull Echo and the professor didn't find a way to get further down. How did the locals manage to get to the surface? Unfortunately the pictures didn't mention this.

Completion
We decided to get back to the surface and explore the other craters. However something caught the attention of Dull Echo: the sand near the wall itself was loose, as if someone had dug it recently.