Somewhere between a huge funhouse, playground and circus, the City Museum of St. Louis may be the most entertaining and interactive urban architectural experiment in the world. And if you are not having a good time, you can always hop on the 10-story slide, a remnant of the structure’s days as a shoe factory (originally designed to send products down the side of the building).

Eclectic from its stylings to its offerings, this unique place features everything from recycled buses and airplanes to giant multi-story slides and artificial caverns as well as more conventional kid-friendly fun in the form of skate parks and ball pits.

Various other imported, salvaged and upcycled oddities can be found throughout, including a vault and safety deposit boxes from a Chicago bank. And the place is constantly changing, being reconfigured and hacked away at by the Cassilly Crew.

What started inside of a derelict structure in the late 1990s has burst from the walls of the building, featuring an array of exterior ‘exhibits’ as well. Visitors can climb ramps, bridges and tunnels to access a high-hanging plane and other repurposed spaces. Up on the roof sits a small Ferris Wheel while a bus hangs over the edge of the building (and of course: people are welcome to climb inside).

The owners boast that they are “always building,” and Gallery Hip summarizes the strange paradoxes of this ever-changing place: “popular among residents and tourists, the museum bills itself as an ‘eclectic mixture of children’s playground, funhouse, surrealistic pavilion, and architectural marvel.’ Visitors are encouraged to feel, touch, climb on, and play in the various exhibits.” Or, as Colossal describes it: “hundreds of feet of tunnels that traverse from floor to floor, an aquarium, ball pits, a shoe lace factory, a circus arts facility, restaurants, and even a bar… because why not?”

Faced with this amazing place, one is left to wonder: would such an unusual endeavor be approved of were it being started from scratch today, or would safety-minded citizens suck the fun out before it got started? Like Adventure Playgrounds, also more popular in an era now past, it is hard to imagine this kind of project getting off the ground, but thankfully there is a precedent: it is hard to argue with the success of the City Museum.