This is the second Gideon Oliver mystery by Aaron Elkins. The dark place is the Olympic National Park in Washington, here the dense canopy hides the sun from most of the forest floor. A few bones in a basket are found by a hiker, and a large footprint is found nearby.
John Lau asks Gideon to provide whatever information he can determine. These are the bones of several people and dead by a number of years, it could be an indian burial site, but there are no Indians in the park. Another sign points to big foot, so the crazies are about.
The story is well-told and a good mystery, throw in a love interest with a park ranger and you have an enjoyable story.