Is it possible to get photo spheres in Google map

Question

Is there any way to fetch available photo spheres (within map bounds) using the Google Maps API?

I am trying to build a map like the one here, but can't figure out where to get the photo sphere data from, or if it's even possible - cannot find anything in the documentation other than standard streetview stuff.

edit: I do know that individual photo spheres can be fetched by their pano_id, but need a more efficient way to get a bunch of them onto my map. Manually creating a pano_id collection isn't an option.