Do you own the water running under your property? If you're not the kind of person who would care one way or another, then read no further (although it is beyond me how anyone could fail to be fascinated by this topic!). Internationally, water law is of more critical importance than most people realize, because it is water rather than oil that causes the most political tension in the arid Arabian Peninsula.

The law concerning underground water is quite fractured in the United States, with several competing legal theories, this one or that one dominating depending on where you live and how much water they have here. Note that I am not talking about underground streams here but diffuse water, the stuff that percolates up to the surface or remains still. This matters more and more the dryer the climate you live in. Following is a very simplified summary of the four major legal doctrines:

1. Absolute Ownership

Finders, keepers. You can pump all the water you want from your own land, even if it squeezes the land of neighboring property like a sponge and leaves them dry as a bone. No slant drilling allowed, however.

2. Reasonable Use

You can pump as much as you can reasonably use for “beneficial uses”. How much is “reasonable” is anybody's guess, but the answer can probably be found in state case law (and may depend on how much money you pay the judge!).

3. Correlative Rights

All owners of property over an underground basin jointly own the underground water and may only use a reasonable proportion of the annual supply. Note that this is different from #2 above, because what is “reasonable” is a lot more dependent ion the total supply of water in the basin – the less there is, the less water you can take and still be reasonable.

4. Appropriation

This one applies to underground streams as well. Under this doctrine, people who don't even own land over the basin can take water out as long as they can do so without trespassing.