St. Louis lost a visionary when Bob Cassilly died Monday at 61 in a construction accident. Cassilly was the man behind The City Museum, a unique venue that brought joy to tens of thousands of kids (and adults).

The City Museum is not a museum, in that it doesn’t contain exhibits full of paintings or historical artifacts. It’s a warehouse that Cassilly turned into a place where kids can play. We’ve taken our daughter many times — even had one of her birthday parties there — as well as my nieces and nephews from out of town who loved it so much that, whenever they visit us, they beg for another opportunity to go.

It’s hard to give you a capsule description of what’s inside The City Museum, but here are a few of the features: enchanted caves, a huge aquarium, a train, tree houses, a ball pit, a shoelace factory, and Circus Harmony (a troupe of young performers who tumble and twirl). And that’s just the inside. On the outside, there’s an area called Monstrocity, where kids climb through cages to get to really long slides or an abandoned airplane, or sit around a pit fire and roast marshmallows. Oh, and there’s a ferris wheel on the roof, near the school bus that hangs off the edge.

By far, the greatest thing about Cassilly’s creation is that it inspires kids to explore and discover on their own. There are no signs or staff telling you where to go next or what to do. The place is full of holes — you can climb down this one into the tunnel under the floor and come up at the other end, or you can climb up through another hole and find yourself on a row of rollers hanging two feet below the ceiling, where you have to pull yourself along until you exit somewhere else. You can run around like a maniac on the curved walls of the indoor skateboard park (where skates and skateboards are prohibited!), or wait in line for your turn on the giant tire swing. It’s up to you to make decisions, figure out what goes where, and then move on to whatever you want to do next.

Cassilly understood the value of teaching kids the importance of exploration, in doing things by trial and error, in getting out from under their parents and just winging it, in the simple joy of discovery. That sense, that curiosity, is how the world ends up with Google and the iPhone and the electric light and men on the moon, but too many 21st century kids lead a life that’s limited. The City Museum removes those limits.

This concept is a little tough for some parents to deal with. They’re so used to hovering over their children to make sure they know where and what they’re doing at all times. But at The City Museum, you have to be willing to say to your child, “Go ahead and explore the caves, then come back and meet me over here at the whale’s mouth,” the spot that’s become the unofficial family re-grouping area.

The first time we took our nieces there, my brother-in-law, an attorney, was stunned that, in our litigious society, The City Museum hadn’t been sued out of existence. Sure enough, kids do fall down and get scrapes and bruises (just like everywhere else), and Cassilly has had to fight off several lawsuits, but the numbers are small compared to the 600,000 who visit each year.  The City Museum isn’t a dangerous place; it’s a wonderland shaped from reclaimed building materials, salvaged bridges, old chimneys, and miles of tile.

The genius behind it, Bob Cassilly, may be gone, but his unique creation will continue to entertain and inspire kids for a long time.  May that thrill of discovery stay with them forever.