The under-construction SR99 Seattle Alaskan Way Viaduct highway tunnel ranks as one of the most ambitious tunneling projects ever attempted in North America. Or so this video shot with a drone on behalf of the Washington State Department of Transportation (WSDOT) suggests. Despite numerous setbacks, Bertha, the tunnel boring machine, has bored more than 1,500 feet since the project’s inception in July 2013.

The Bertha tunnel-boring machine. Image source: Dragados USA The SR99 measures 57.5 feet from top to bottom and when completed, will house two stacked roads containing two lanes each. According to the WSDOT, the two-mile-long tunnels will also support state-of-the-art ventilation, electrical, fire-suppression, CO2 monitoring systems and more.

The department also lists Bertha as the biggest tunnel boring machine ever manufactured. While its excavation diameter measures 57.5 feet, its shield and cutter length are listed as 49.17 feet. Its total length is just over 256 feet.

Construction of the two-mile-long tunnel stalled in December 2013 at 1,083 feet due to mechanical failure. With the original completion date of December 2015 now past, the new date set for the project’s finish is early 2018.