Something is rumbling under Seattle. Hint: she’s 7,000 tons, 316-feet long, 57.5-feet tall and has the thrust of 126 Boeing 747 airplanes. If you guessed Bertha, the world’s largest tunneling machine which is busy digging the world’s largest diameter bored tunnel, you’re right! And now you and your crew can learn all about Bertha and the ground she’s digging through at the Milepost 31 Information Center. Read on for the dirt!


photo: Helen Walker Green

X Marks the Spot 
Milepost 31 marks the spot where the SR 99 tunnel travels under Pioneer Square. At this spot, you won’t just learn about Bertha, you’ll learn about the part of town she’s boring through, including the glacier, earthquake and tsunami that shaped the land and the people who shaped it too, from native civilizations to the pioneers.

We Dig Bertha!
When you enter the information center, be sure to grab a scavenger hunt sheet. Kids can search for objects around Milepost 31 and earn a commemorative Bertha sticker as a reward for completing it. Also, be sure to view the scale model of Bertha – we guarantee you’ll be blown away by everything that’s inside and outside the tunnel boring machine. Psst! Don’t miss the fascinating video that will give you a Bertha’s-eye-view of the tunneling process.


photo: Helen Walker Green

Tracking Bertha
If you want to keep track of where Bertha is traveling, you can check out Bertha’s mining progress on an aerial map in the information center and see exactly where she is on the day you visit. Did you know Bertha tweets? Follow her at @BerthaDigsSR99.


photo: Helen Walker Green

What does toothpaste have to do with the SR 99 tunnel project? Well, the key to Bertha’s tunneling success is her rotating cutter heads that turn the soil she’s churning into the consistency of toothpaste, making it easy to push through. At Milepost 31, you can view the three most common types of soil Bertha has to “toothpaste-ify.”  A tunnel boring machine sure would be handy for digging in the yard, don’t you think?

Milepost 31 is definitely a hands-on experience for kids and parents alike. Kids can touch the exhibits and even sit in or on them. A must-see highlight is Bertha’s Last Stand, a soapbox derby car inspired by Bertha. And yes, kids can get inside the car and pretend to drive it!  Psst… this car was runner-up in the 2014 Red Bull Soapbox Race in Seattle.


photo: Helen Walker Green

Tunneling to the Future
Bertha may not go as fast as her soapbox namesake, but one day in the not too distant future, you and the kids will be driving through this tunnel. For now, you’ll have to use your imagination!


photo: Helen Walker Green

Take a Souvenir
Some kids would love the chance to investigate a pile of construction site rubble (we’re looking at you, digger fans!). But this isn’t just any rubble – you may have driven over it many times – it’s all that’s left of the southern section of the Alaskan Way Viaduct, demolished in 2011. The concrete was crushed to use for tunnel construction, but several chunks were kept for the public to keep as souvenirs. So before you leave, help yourself to a small piece of the viaduct to take home as a keepsake.


photo: Helen Walker Green

Time Tunnel
One of the most fascinating parts of Milepost 31 is the chance to see the history Bertha is uncovering as she burrows through the ground. Tall tubes show the layers of history in the dirt and a huge rotating wheel shows the passage of time. Psst! At 12-feet below, you’ll see sand, clay and gravel left behind by a huge melting glacier; at 10-feet below, you’ll see shells from when the area was a beach. Look further up the tube and you’ll see wood chips and sawdust from Henry Yesler’s steam mill and burned wood and rubble from the Great Fire of 1889. Fascinating!


photo: Helen Walker Green

A Tale of Two Berthas
Want to know how Bertha got her name? Bertha was named by Seattle school kids in a competition. She’s named after Bertha Knight Landes, elected Mayor of Seattle in 1926 and the first woman mayor of a major American city. Both Berthas have been movers and shakers of Seattle!

Boring Bertha
We couldn’t resist making the obvious pun—even Milepost 31 has it on their sign—but this information center is anything but boring. We’d recommend it for kids 8 and up as there’s only a couple of things to really capture a little one’s attention (although the Littles could probably spend hours in the soapbox car). Speaking of time, expect to spend anywhere from 15 minutes with younger kids, to 45 minutes with the bigs inside the information center. Grownups could easily spend an hour or more. The best part? Milepost 31 is FREE, so you can always come back if you really dig it (sorry, we couldn’t resist!).


photo: Helen Walker Green

Bertha Up-Close and Personal
Still want more Bertha? Take the self-guided tour that begins at Milepost 31. You can walk or bike along a paved path and view the heart of the construction. Pick up a map at Milepost 31 or download one here. Psst! Bertha fans 13 and older can go on a free tour to see tunnel construction up close. This is where you’ll get great views from the Tunnel Construction Viewing Platform (a section of the viaduct that’s been permanently closed to traffic). Tours last one hour and begin at Milepost 31. Just remember… you will be in an active construction zone, so that’s why younger kids aren’t allowed and guests ages 13-17 must be accompanied by a parent or guardian. You can find tour times and sign up online.

bertha-milepost-31- signs

photos: Helen Walker Green

Explore Pioneer Square and Beyond
Pioneer Square is a great area to explore with kids. After checking out Bertha, take your crew for a bite to eat at one of the food trucks in Occidental Square Park, cruise over to the Seattle Pinball Museum for some free play, enjoy a game of table tennis or head to Seattle’s Chinatown-International District for lots of family fun.


photo: Helen Walker Green

Milepost 31 – SR 99 Tunnel Project Information Center
211 First Ave. S.
Seattle, Wa 98104
Online: or map it

Cost: Free
Hours: Tues.-Sat., 11 a.m.-5 p.m. (closed on state holidays). Open until 8 p.m. on the first Thursday of each month during the Pioneer Square Art Walk.

ADA: Milepost 31 is ADA accessible. There is one step at the main entrance and a portable wheelchair ramp is available for visitors who may need it. Please ring the buzzer and Milepost 31 staff will be happy to assist you.

Parking: Visit WSDOT’s parking website to help track down a parking spot in Pioneer Square. The site includes rates, hours and directions to various lots and garages.

Do you dig Bertha? Have you visited Milepost 31? Don’t be a bore, tell us in the Comments below!

— Helen Walker Green