Dan reveals his secret with this explanation:
"Thanks for your interest! If you don't own a 220 degree fisheye lens (they exist!) and a helicopter then you'll need some software to make these. I stitched this together using a program called ptGUI which can do it automatically, although to really control the outcome, I use the technique below. There are several other programs that can do this and many digital cameras will include some software for free. It can be done in Photoshop or Elements but it takes a while. I usually make a cylindrical panorama in ptGUI(ie flat, but the ends can blend into each other), then I warp it into a circle in Photoshop by first changing the aspect ratio to 1:1, rotating the image 180 degrees and then using a photoshop filter called Polar Coordinates. To get this bit right you may need to include dead space above and below to control the degree of distortion- in this case, trying to emphasise the background and minimise the foreground. After a bit of touching up it usually takes about 3 or 4hrs before I get something I'm happy with."
You can see more of Dan's work, including more panoramas at his UKClimbing.com gallery, here