Groundwater is a natural resource supporting industry, agriculture and water supply worldwide. It is especially important in drier, water-limited parts of the world, where it can provide a resilient water source in times of drought. Despite this economic importance, we do not know how the replenishment of groundwater (technically known as groundwater recharge) relates to climate phenomena such as El Niño and La Niña. This is increasingly important as we adapt to climate change and associated climate extremes, such as the recent rare occurrence of three consecutive La Niña years. This knowledge gap can be filled by using caves, uniquely situated between the land surface and the groundwater, as observatories of groundwater recharge in the past, present and future. This lecture will explain how we can generate new knowledge by combining the analysis of cave stalagmites to understand when recharge occurred in the past; underground hydrological monitoring today to work out how much rainfall is needed and the associated weather and climate patterns; and climate and hydrological modelling to identify when this replenishment will occur in the future. Using these approaches, we can identify the climate conditions most likely to lead to the rainfall recharge of groundwater, and how this recharge of groundwater will change with future climate change. This new knowledge can be used to help identify where groundwater can be sustainably used for water supply and industrial use in the future and to mitigate the impacts of longer and more intense droughts that are predicted with climate change.