I decided to build a simple recessed cellar in my yard that would serve a variety of functions, including rainwater storage, root cellar and space for vermiculture (worm bins). All of these require relatively cool but not freezing conditions, so digging into the ground was a good way to accomplish this. It seemed like the easiest approach would be to build this primarily with earthbags.
First I calculated how large it needed to be in order to hold the two large (275 gallon) water tanks I intended to buy, as well as provide enough space for produce and worms. It was a tricky business to fit all of this in the space available, especially because I had to work between existing underground irrigation pipes.