古地下水，又称原生水或化石水（英语：Fossil Water），是一种很长时期密封于蓄水层里的地下水。数千甚至数百万年前因为周围地质的变化，某些地下水从此被密封于“化石含水层”里，而不再受到地下水循环的补充，就成了所谓的化石水。

努比亚砂岩含水层系统是其中最显着的化石水储量之一。此外，在撒哈拉沙漠，喀拉哈里沙漠及美国大平原的地下也存有化石蓄水层。

“Fossil water” or paleowater is a somewhat broadly-used term to describe water that has been contained in some undisturbed space, usually groundwater in an aquifer, for millennia or longer. Fossil water could also describe ancient water on other planets, or water sealed in subglacial lakes such as Antarctica’s Lake Vostok. “Fossil groundwater” can be defined as “water that infiltrated usually millennia ago and often under climatic conditions different from the present, and that has been stored underground since that time.”