This is an intereting question! I'm not really sure that thinking of sunspots in terms of "depth" is quite accurate, because they are regions of intense bundles of magnetic field lines extending from below the surface outwards. They appear dark at the surface because the radiation doesn't make it through as well (due to the large fields which inhibit it from doing so). So, although they appear as deep holes, they really aren't that much different from the rest of the surface (in terms of depth)- the entire surface contains field lines extending through from below- it's just that sunspots have a much more concentrated amount of field lines. I hope this makes sense! One of the other solar scientists who study sunspots might have a better answer.

Holly

One instrument on the SOHO spacecraft has allowed us to look at what's below a sunspot. By analyzing perturbations of the surface of the Sun caused by sound waves trapped in the Sun, scientists have measured the flows beneath sunspots. They were surprised at how shallow the sunspots were! You can see the picture in a press release at http://www.gsfc.nasa.gov/...0010919sunspot.html. The technique of analyzing these surface perturbations from sound waves is called "helioseismology." Paulett