Seattle Public Utilities and King County Wastewater Treatment Division are building a tunnel to store stormwater and sewage in heavy rain until it can be treated.

https://www.king5.com/mobile/article/tech/science/environment/boring-machine-mudhoney-finishes-seattle-stormwater-project/281-aa3eaf67-5c3e-4c1e-861c-672cd156cc54