“Another story about alleys? You’re going to have a PhD in them!” my sister said, amused, when I got my latest assignment. It was a piece about depaving alleys as part of the urban-greening movement. (The feature, for YES! Magazine, was just published this week.)

The backstory on my interest in back routes: About three years ago, on a bike tour of Montreal, I realized that its green alleys took travelers like me backstage, giving us a glimpse of more hidden local life. We got invited for a backyard glass of wine, and to look at a private sculpture garden, among other alleyside treats. Montreal’s “laneways” also host trick-or-treating, block parties, pop-up restaurants, I learned. I described the scene for the readers and intrepid travelers of the New York Times.

From there the interest in these tiny spaces has only mushroomed. Alleys are used in interesting ways around the world, I learned as I tunneled deep into the subject for the Atlantic’s CityLab). In Europe, cities like Stockholm and Istanbul retain these sort of medieval warrens. In China, Beijing is famous for its hutongs, or mysterious walled alleyways. And in Australia, Melbourne’s laneways are old delivery routes that have become rocking bar and foodie scenes. (Hiya, ACDC!)

Here in the U.S., city designers purposefully laid alleys out until World War I, according to the late scholar Grady Clay. (Clay’s book Alleys: A Hidden Resource became a rich source for me.) Alleys were used for horses and carriages, out-back garbage pickup, and improvised living areas, notably for immigrants and newly freed slaves after the Civil War, Clay reported. After World War II, however, Americans wanted to show off their boat-sized cars out front. By the 1960s, the American Planning Association considered alleys obsolete, “one of the advances that has been made in the Motor Age.”

These days alleys are hitting up against fresh trends. Millennials want walkability and community, and cities are getting both denser and greener. So places like Chicago and Nashville have realize you can mitigate climate change effects like flooding with better alleyways. I’ve checked out some exciting things happening along American alleys (and reported on them for more outlets, like Sierra Magazine). Miami’s high-end retail Design District features showplace alleys by a rising young architect. Flood-prone Nashville is planting alleyside rain gardens. And a Detroit brewery joined in on a test project, the Detroit Green Alley, that was so successful the Midtown area wants to dedicate itself as an Alley District.

Along the way certain experts on the subject have really deepened my appreciation of alleys. They include Michael Martin, a landscape architect at Iowa State, who found through multiple studies starting in the 1990s that alleys could be community connectors. And Daniel Toole, the millennial architect behind the Design District project, has studied and thought deeply about alleys around the world.

I’m excited to see what cities like Chicago, Seattle, Nashville, Montreal, and Detroit do with their green alleys next. If you really want to geek out on alleys like I have, here’s an interview I gave on them to NPR’s Kansas City affiliate. Or just take a back route on your next walk. See what’s hidden there, and imagine what more the space could be.