Milepost 31

Milepost 31



Milepost 31 is an award-winning information center that highlights the people and projects that shaped Pioneer Square, and provides an inside look at the SR 99 Tunnel Project. There, you'll find more than just construction photos and brochures. You’ll find history, artifacts and interactive exhibits designed to broaden your understanding of the land beneath you. You’ll explore the neighborhood’s changing landscape, from earth-moving efforts of the past to the massive tunnel project that will soon move State Route 99 underground and reconnect Pioneer Square to the waterfront.

Location and hours

211 First Ave. S., Seattle
Admission is free.
11 a.m. to 5 p.m. Tuesday - Saturday (closed on state holidays) 
Open until 8 p.m. on the first Thursday of each month during the Pioneer Square Art Walk - see below for details. 

Exhibit details

Visitors to Milepost 31 can browse through four sections:

You Are Here: Similar to the "you are here" points on maps, this section orients visitors to Milepost 31. It tells the story of the land upon which you are standing from the perspective for several different historical figures.

Moving Land: This section examines how the natural forces of glaciers, earthquakes and volcanoes have transformed Seattle's landscape during the past 20,000 years. Visitors will also learn about our own effects on the land, from the filling of the tidelands in Pioneer Square to the various regrade projects across the city.

Moving People: This section tracks transportation over time, with an emphasis on Pioneer Square. Visitors will see how people-moving has changed - and in some cases stayed the same.

Moving Forward: This section is all about tunneling. Visitors will learn about the history of tunneling technology, tunneling in Seattle and, of course, the SR 99 Tunnel Project. In addition, exhibits show visitors how the project - along with the Elliott Bay Seawall Replacement and Waterfront Seattle - will transform the future of Pioneer Square.

Why the name “Milepost 31”?

Mileposts mark progress. They help you track where you are on your journey, reminding you of the places you’ve passed through on your way to somewhere else.

But what if a milepost is so interesting that it becomes a destination? Located on SR 99 at the western edge of Pioneer Square, Milepost 31 is that kind of place. It marks a spot on the highway, but it also marks the spot where, before mileposts existed, mile-thick glaciers gave way to native civilizations. It’s where Seattle’s first neighborhood saw the rise of the city’s most notorious stretch of highway - the SR 99 Alaskan Way Viaduct - and where crews building the world’s largest diameter bored tunnel to replace the viaduct will first cross into the soils beneath Pioneer Square.

Events and activities 

First Thursday speaker series 

WSDOT hosts a monthly speaker series in conjunction with the First Thursday Art Walk in Pioneer Square.

Thursday, May 5 
Milepost 31, 211 First Ave. S., Seattle
Admission is free
6:00 p.m. - 6:30 p.m.

Puget Sound’s Mosquito Fleet: Washington’s First Passenger Ferries

When the first settlers arrived in the Puget Sound region, there were no busy highways or railroads to get around the heavily forested coastlines. The easiest way to transport goods and people was to travel by water on a swarm of steamships known as the “Mosquito Fleet.” Join Joe Baar, a volunteer at the Puget Sound Maritime Historical Society, to discuss how this fleet of steamships transported nearly everyone and everything throughout Puget Sound and its many inlets and rivers.

Joe Baar volunteers at Puget Sound Maritime Historical Society, mostly assisting with research enquiries. He dates his obsession with maritime matters to the 1950s, growing up on the beach south of the Fauntleroy ferry dock.  


If you have questions about Milepost 31 please call the program hotline at 1-888-AWV-LINE, which is answered by staff between 8 a.m. and 5 p.m. on weekdays.

Visit Milepost 31.


Celebrate Seattle's past and future.


Come check out a model of the world's largest tunneling machine!


Click to sign up for a tour of tunnel construction from the viewing platform