Thanks for the constructive replies, guys. The reason for going underground is for visual appeal. We don't have a lot of back yard...most of it is pool...and I don't like the filter and pump in full view.
I haven't studied the grade to see if I can get good natural drainage from that depth. It may need a sump pump...that would be a complication.
The alternative is to build a pump house. Any experience on that would be appreciated.
Don