This is known as the Little Planet effect. There are many ways to render it, and usually, the sky is on the outside and the ground, trees, buildings, etc. inside. But in this image they inverted that. This is what they refer to as a "tube" projection in the above link:
Some fisheye lenses use a stereographic projection to capture a wide-angle view. Compared to more traditional fisheye lenses which use an equal-area projection, areas close to the edge retain their shape, and straight lines are less curved. However, stereographic fisheye lenses are typically more expensive to manufacture. Image remapping software, such as Panotools, allows the automatic remapping of photos from an equal-area fisheye to a stereographic projection.
The stereographic projection has been used to map spherical panoramas. This results in effects known as a little planet (when the center of projection is the nadir) and a tube (when the center of projection is the zenith).