You think you know a city – its storefronts and alleys and long-standing pizza joints. You think you’ve figured it out, having spent years on its sidewalks and in its bars talking about all the changes over at North Dam Mill or the new coffee shop or the hot cross buns from Reilly’s that you longed for during those years you were away at college.

You think you know a city. And then you find out it’s been hiding an underground lagoon from you for the last century.

It’s true. Biddeford has places you’ve probably never seen.

But Heart of Biddeford’s Secret Spaces/Historic Places tour returns this weekend to give curious tourgoers a peek into some of the city’s under-lock-and-key locations.

The self-guided tour takes place on Saturday, May 31 from 11 a.m. to 3 p.m. and includes rarely seen spots both above and below the city, like the mill lagoon and the City Hall clock tower. This year there’s an additional option to take a trip down the Saco River on Captain John’s Charters, arriving at University of New England, then returning by bus to enjoy an historic tour of Biddeford Pool.

Tickets are $20 for the Downtown/Lagoon Tour or $40 for the Downtown/Lagoon plus Boat/Bus/UNE Tour.

Attendees will finally get to know Biddeford inside and out, from high up to underground. Here’s a sneak peek at some of the top-secret tour stops:

Mill Lagoon: Under the #10 Mill – a place once flooded by cotton and Vellux blankets – is a hidden lagoon. Redirected water from the Saco River flowed through the underground tunnels beneath the building and the space was kept under tight wraps – mill workers rarely even saw it.

The water stopped flowing decades ago when that newfangled electricity took over, but the huge subterranean room still exists. The space is still and quiet now, aside from the echoing trickle of ground water and, possibly, a pirate skeleton and Chunk from Goonies hiding out in one of the dark tunnels.

City Hall Clock Tower: Alternating and incongruous staircases lead the way up to the the City Hall clock tower where the historic clock is on “sleep status” until funds are raised to repair it. Until then, it’s 1 pm all the time.

The clock was built in 1895 by Howard Clock in Boston, but was mechanized in the 1980s. The bell stopped ringing a while back but the clock itself went kaput just a few years ago. The city is hoping to return it to its original state when they’re able to raise the money to do so.

There’s also some entertaining writing on the walls up there, including:

“O.J. Simpson 1982

I didn’t do it”

Additional stops, with descriptions from Heart of Biddeford:

Former St. Joseph’s Church Rectory: No longer part of the Church, this historically and architecturally significant (and breathtaking) rectory will be on display, with help from students and a tour guide.

MotorLand Vintage America: Visit the classic cars and have your mind blown by Caleb Delano, a young intern who has discovered incredible stories to tell about the link between license plates and history.

River/Biddeford Pool/UNE Tour: While enjoying the scenic Saco River, you’ll hear about its role in our area’s history and see many of its historic landmarks. Once on campus at UNE, you’ll see Champlain’s plaque and learn about this important landing. The tour through Biddeford Pool will show the beauty and historical significance of this beautiful part of our community.

The Former Maine Energy Recovery Company Site: Meet the City’s Economic & Community Development Director, Daniel Stevenson, and learn about the City’s plans for this incredible site on the Saco River. Biddeford High School students will give you history of the Saco River and describe their dreams for its future.

The tour is self-guided, but there will be tour guides at each location to offer some “informative, sometimes dramatic, narratives.” Tickets for the boat/bus portion of the event are limited and need to be reserved in advance. Get tickets: www.brownpapertickets.com. Proceeds benefit Heart of Biddeford.

FMI: Secret Spaces, Historic Places: 11 a.m. to 3 p.m. May 31 | Tickets are $20 for the Downtown/Lagoon Tour, or $40 for the Downtown/Lagoon + Boat/Bus/UNE Tour. | www.heartofbiddeford.org or 286-8520.

Purchase tickets: www.brownpapertickets.com or at the Heart of Biddeford office, 205 Main Street, Ste. 103, Monday-Friday, 10 am-2 pm