This is a tank built by Eric for his solar space heating system. The pictures were submitted by Eric (ericjeeper) to the http://HomesteadingToday.com Alternate Energy forum, and copied here with permission -- thanks Eric.
The tank has no structural walls other than the dirt -- can't get much simpler than that.
The tank is lined with 6 inches of extruded polystyrene (pink) foam. Early indications are that the heat loss from the tank is low.
Obviously, you need to have soil that allows this to work -- cohesive enough to make good walls, and not to wet.
The liner is PVC, but EPDM would likely also work well.
The inside dimensions of the tank are about 36 x46x84 inches for about 600 gallons.
There are a few more pictures of Eric's project here: