Lately I've been looking for examples of good, large, detailed 3D skyboxes, because I need inspiration for them. I noticed that Valve's own 3D skyboxes tend to be rather simplistic or even ugly (with the exceptions of the ruined City 17 in the first map of EP2, and the skybox of the finale of No Mercy in L4D, these look pretty nice. One particular butt-ugly Valve skybox is the C17 ''skyline'' seen from atop the Nexus in HL2 though), and many custom map skyboxes are even worse, but I know that somewhere under this pile of shit is a true diamond.
So now I ask you, what is the most beautiful 3D skybox you've ever seen on Source, whether it's in an official game map or a custom map?
Include screenshots if you can.