During the high middle ages, the piston pump and the (human or animal powered) treadmill were introduced. These allowed medieval miners to mine deeper and keep the mines (reasonably) dry. Roman mines were re-opened and new mines, especially in northern Europe became more productive and able to produce longer since the miners could dig deeper.

An excellent source on this is the De Re Metallica (link to a translated version on Gutenberg.org) which also includes many very well-done copper carvings illustrating mining in the first half of the 1500s.

Here's one of the illustrations, showing how a treadmill and piston pump are used to keep a mineshaft dry.