People who love and are obsessed by history, particularly geography-based history, have a new tool with which to play.

Historypin is a collaboration between a U.K.-based Web site and Google. It’s now a smartphone app that enables users to not only see how a location looks now, but to annotate and personalize it with photos, videos, audio clips and stories, then share those bits of history with everyone else. You can retain ownership of your media — Google says it’s just a site for sharing, like YouTube.

Here’s Google’s announcement, and here’s the map of downtown D.C. (you can slide the timeline so you’re looking at what’s available for the years between 1840 and now.)

For example, pick out an intersection in Washington, let’s say 16th and K streets NW. Nowadays it’s an office building, but back in the 1920s, there was an ivy-covered house called the Little Green House, which as author Garrett Peck wrote in his 2011 book “Prohibition in Washington D.C.: How Dry We Weren’t,” was “the headquarters of the Ohio Gang, President Warren Harding's friends who looted the national treasury.”

The Little Green House on K Street in the 1920s. (LIBRARY OF CONGRESS)

You can overlay it on an existing Google Street View map, and later this year, Google promises new features such as embed tools that let you add the app to your own site. People who are very into genealogy would find this useful, as well.

It’s been in beta for a year, but just officially launched this week. Local historical societies have started to partner with them, so if you’re into Peoria, Ill. history, expect more soon.