Silver, he writes, does not occur in native form and has to be mined, usually occurring with lead ores. Spain produced the most silver in his time, many of the mines having been started by Hannibal. One of the largest had galleries running up to two miles into the mountain, while men worked day and night draining the mine in shifts. Pliny is probably referring to the reverse overshot water-wheels operated by treadmill and found in Roman mines. Britain, he says, is very rich in lead, which is found on the surface at many places, and thus very easy to extract; production was so high that a law was passed attempting to restrict mining.