A deep hole, in the earth, that a petroleum or natural gas company drills in the hopes of locating a new source of fossil fuel. An exploratory well represents a risk for the company drilling it, because it is not known, before investing in the well, how much oil or natural gas it might contain. The well may turn out to be a profitable new source of fossil fuel, or it may contain noncommercial quantities of fuel that aren't worth extracting; in the latter case, the well may be plugged and abandoned.