After about 16 months I finally found my 500th geocache which I specifically picked for this milestone. Fear Factor: Up the Rat Hole by the DAK Girls. It is difficulty 2 and terrain 5 and was physically challenging.
We (myself and NavalAv8or) had to get into the drainage tunnel which is only about four feet high and walk at least 35 yards in. You can see the next challenge on the cache page.
This picture was taken as I exited the tunnel while trying to keep my feet dry. I definitely made a good choice for a big milestone. Here is my full log entry.