Any void, whether of tectonic or dissolutional origin, that is smaller than the accepted defined size of a conduit. Sub-conduits originate under inception conditions and enlarge during gestation, but many fail to achieve larger dimensions when drainage later becomes concentrated along preferred routes. In most cases, however, they will continue to function as part of the micro-fissure, or percolation, system within the rock mass. Sub-conduits are an essential part of a continuum of void sizes that extends between microscopic discontinuities and the largest tube passages [9].