Sinkholes
Sinkholes form when the land above a cavern collapses into that cavern. In a karstic landscape, this occurs when underground limestone dissolves due to contact with groundwater containing dissolved CO2, leaving a cavern that eventually collapses, forming a sinkhole.
In the above image, we can see a sinkhole formed through different means. Bellingham, WA is not a karstic environment, which means this sinkhole is unlikely to have been formed through the dissolution of limestone. Rather, what may have happened here is an old mineshaft filled with water, weakening the structure, and resulting in a collapse, forming a sinkhole.