Underground waters, when not flowing in a defined course,148 but existing merely as percolations, may be diverted,149 although by so doing the wells of adjoining landowners may be injured.150 Upon the same principle, in working a mine, subterranean waters may be drawn off from the surrounding land without incurring liability.151 But underground percolations must not be fouled by the introduction of foreign substances.152 No easements can be acquired by prescription in subterranean waters, because the user necessary to acquire such rights would be unknown, and therefore not adverse.153