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
Milepost 31 will be closed Thursday, Nov. 26 and Friday Nov. 27 for the Thanksgiving holiday. It will reopen on Saturday, Nov. 28. 

Events and activities 

For more information visit our events and activities page.

First Thursday speaker series

We host a monthly speaker series at Milepost 31 to give visitors more insight into our work to replace the viaduct. The event is held in conjunction with Pioneer Square's First Thursday Art Walk

Thursday, Dec. 3
Milepost 31, 211 First Ave. S., Seattle
Admission is free
6:00 p.m. - 6:30 p.m.

Join us at 6 p.m. on Thursday, Dec. 3 as geologist David B. Williams gives a special guest presentation, The Protean City: Reshaping the Seattle Landscape. Since settlers first arrived in Seattle, the city’s citizens have altered the landscape with an unrivaled zeal. Seattleites have regraded hills, reengineered tideflats, and replumbed lakes to provide better locations for businesses and easier ways to move through the city’s challenging topography. Williams’ presentation will focus specifically on the area around Pioneer Square, looking at how early Seattleites altered the land and how that still influences the modern city.

Williams is a freelance writer focused on the intersection of people and the natural world. This talk is based on his recently published book, Too High and Too Steep: Reshaping Seattle’s Landscape (University of Washington Press). Previous books include Stories in Stone: Travels Through Urban Geology and The Seattle Street-Smart Naturalist: Field Notes from the City. Williams also works at the Burke Museum and maintains the website

After the talk, be sure to leave enough time to explore the rest of the First Thursday Art Walk in Pioneer Square. Milepost 31 is open Tuesdays through Saturdays from 11 a.m. to 5 p.m., and stays open until 8 p.m. on First Thursdays. Free parking is available for First Thursday art walk patrons in Pioneer Square. Please visit for more information about participating garages.

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.


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.


Seattle's amazing tunnel boring machine model on display at Milepost 31.


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