The First Restatement of Property provides that “‘property’ . . . denote[s] legal relations between persons with respect to a thing.” The “thing” considered in this article is the subsurface of real property, and the legal relationship involved in the extent to which persons may deal with invasions of their right to exclude trespassers is a fundamental incident of property ownership, this right, like other incidents, neither is nor should be absolute. The precise focus of this article is to consider how courts have treated subsurface invasions and to argue that the Second Restatement of Torts, which states that any subsurface intrusion is actionable, should be revised. The right to exclude trespassers from the subsurface of real property should be much more limited: subsurface trespass should not be actionable whenever the trespasser’s subsurface intrusion accomplishes an important societal need (including private commercial needs) if the subsurface owner suffers no actual and substantial harm. And because courts have largely refused to find harmless deep subsurface invasions actionable, the ALI should consider whether the Second Restatement of Torts, which essentially embraces Lord Coke’s ad coelum doctrine, accurately reflects the trend of subsurface trespass case law.