  The natural gas and hydrogen storage industries have experience creating
huge, pressurized underground spaces. The most common of these is "solution
mining", a method for making brine-filled cavities in salt formations. Unlike
conventionally-mined underground spaces, these spaces are (a) inexpensive to
construct and operate, (b) naturally serve as pressure vessels, at size scales
impossible to construct in a conventional lab, and (b) permit safe use of
flammable and/or toxic materials. If various engineering challenges could be
met, solution-mined caverns would allow unprecedentedly-large high pressure gas
TPCs. Lined rock caverns (LRC) may permit high pressure TPCs of considerable
size in more conventional spaces. In this whitepaper, we review some of the new
physics opportunities available in these caverns and suggest an R&D program
needed to realize them.
