If you are building in the ground, I would suggest you check out my site "Pipe Siphon by Apollo". Ground temperature runs at a more consent temp stays closer to the average yearly temp. A lot less temperature swing over several days not in a few hours, I only run a 500 watt heater about 2 weeks a year. Stays above 57 degrees during the coldest days of winter and has never got too warm in the hottest days of summer.

I think deeper is better for temperature control & less surface area for water loss.