Just past the new Olallie Trialhead, there was a fill that Chicago / Milwaukie put in get the small valley a creek flows down, up to the railroad grade.  They used timbers to create a structure, like coffer dams, that helped to keep the steep sides of the fill in place.  Many decades later those verticle timbers have almost completely decomposed, creating a pattern of very deep potholes.  Thank you to State Parks staff for trying to keep up with filling them in, but they sometimes reappear after some heavy rainfall.  There are two new deep potholes in that area, and on my way up to the tunnel, I noticed somewhere around the Hanson Creek and snowshed areas, there are some new deep potholes showing up there as well.  Bicyclists be aware, those potholes could ruin ones day with a crash and or a broken bicycle.