Someone just asked me how to adopt a cache and I thought I would share the answer with everyone.

GC.com now has a quick adoption system. Both cachers have to agree to the adoption.
Cacher1 - Owns the cache now
Cacher2 - wants to adopt the cache

Cacher1 goes to http://www.geocaching.com/adopt/ and enters the GC number of the cache that he owns.
Cacher1 tells Cacher2 to for to the same web site and enter the GC number.

Cacher2 now owns Cacher1's cache.

If you want to adopt someone's cache and you cannot contact them because they don't respond to email, then that is a lot more difficult.

You can try contacting the local reviewer, Cache-Advance http://www.geocaching.com/email/?guid=633aceed-f63d-4c6e-8c13-063d7558032a and explain what attempts that you have made to contact the cache owner.