Much of the world relies on groundwater as a water resource, yet it is hard to know when and where rainfall replenishes our groundwater aquifers. Caves, mines, and tunnels that are situated above the groundwater table are unique observatories of water transiting from the land surface to the aquifer. This paper will show how networks of loggers deployed in these underground spaces across Australia have helped understand when, where, and how much rainfall is needed to replenish the groundwater.