The airshaft to the #4 seam on Cougar Mountain was dug around 100 years ago. The purpose of the shaft was to allow good air into and later to pull bad air from the mines below.
The shaft follows the Number 4 coal seam and is over 1200 feet deep. The shaft was connected to the Primrose Mine by a rock tunnel to the northwest. The location of the air shaft was to allow access to both coal seams.
In the 1920's the shaft was covered by a fan house similar to the one pictured below.
Sites such as this are unique in Washington as almost no coal mine shafts or entrances have been preserved for safe public viewing.
Source: Cougar Mountain Interpretive