The State Route 99 tunnel, also known as the Alaskan Way Viaduct replacement tunnel, is a bored highway tunnel in the city of Seattle, Washington, United States. The 2-mile (3.2 km), double-decker tunnel carries a section of State Route 99 (SR 99) under Downtown Seattle from SoDo in the south to South Lake Union in the north. Construction began in July 2013 using "Bertha", at the time the world's largest-diameter tunnel boring machine. After several delays, tunnel boring was completed in April 2017, and the tunnel opened to traffic on February 4, 2019.