Root cellar

I am wondering if it would be possible to make a small root cellar out of a 1500 gallon plastic septic tank?

Maybe, if you bury it.  The keys to a root cellar are getting it as close to 34F as possible without freezing, keeping it very humid (95%) and having some ventilation (ideally a pipe in that's low and an exit that's high).

Mother Earth News had an article on a septic tank root cellar several years ago. Except they recommended a concrete septic tank. Personally, I'd be cautious with plastic, since it tends to dry out and crack. That aside, it sounds doable. The article will have some specifics to consider.

Yes, IF you never ever get a high water table or drill enough holes it will flood instead of float when the water table gets that high.

As has been commented. It needs to be buried. I would set it on a good base of gravel to address the potential water issues.  Of course, the next  issue is how deep to put it. I would have it safely below the frost line.  Then there is access. I would make sure the access is large enough, will not let ground water in, and is protected from the weather.  I would not want water coming in. Also, I would limit undesired heat and cold entering with an insulated access door. The door would not have to be elaborate. Finally, it should have a lock on it for children and other pests.

I imagine that a complicated formula could be generated to determine how deep to put a root cellar relative to the frost line and the normal winter temperatures in your area.  Firstly, the frost line is very specific to a spot.  If there is snow on that spot, there might not even be any frost.  If it's a plowed driveway, the frost will get to full depth.

If you put your root cellar completely below the actual frost line in the location, I'm guessing the deeper earth temps will heat it up to 40 degrees or so (depending on a dozen factors).  I suspect that putting the cellar shallow enough that the top part is in the frost may help the cellar get closer to 34 degrees.  But frozen condensation becomes an issue.  

Air circulation and ventilation affect this greatly as well.

I clearly don't know the answer but I just wanted to mention that depth could be more important/complicated than you'd think.  Maybe...