Before Street View and PhotoSpheres and Local Guides programs with millions of user-submitted photos, there was Panoramio. The site launched in 2005 as a way for users to share geotagged photos around the world and was later acquired by Google in 2007. Its Google Earth and Maps layers boast nearly 100 million user-contributed images, with many locations around the globe offering more pictures than what's available from Maps user submissions.
However, back in 2014, Google had decided to shut down Panoramio and fold it into Maps. That decision was met with some push back from the community, and thus resulted in the delay of the final doom and Google working to implement more social features and contributions in Google Maps to make the Panoramio shutdown more tolerable.
And we now have an official word again: November 4, 2016. Starting that date, no new users will be allowed, and existing users won't be able to add new photos, likes, or comments. The service and the photos will remain online until November 2017, however, so users have plenty of time to migrate their data.
The Panoramio FAQ page (linked below) details how you can completely export your data from Panoramio if your account isn't linked to Google. If you have linked it, you can grab the data using Takeout, but your photos will also be automatically synced to the new Google Album Archive for free (they won't count against your storage) and to Google Maps if you were using your name to share them (and not your Panoramio username).
There are a few more quirks to understand if you're a Panoramio user, so do check out the FAQ below for all the details as well as the GEarth Blog for a few more notes about the concerns of Panoramio users.