钻井是为了抽取像地下水，天然气，石油之类的天然资源而在地面上钻洞的过程。为了勘探地下自然资源的钻探称作地上凿洞钻探或者钻探。

最早的水井可能是用手在地下水位靠近地面的地方浅浅的挖坑。现代的钻井技术是利用长排种机轴。排种机轴钻出的洞比用人工挖的洞更窄而且更深。

Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, brine, natural gas, or petroleum, for the injection of a fluid from surface to a subsurface reservoir or for subsurface formations evaluation or monitoring. Drilling for the exploration of the nature of the material underground (for instance in search of metallic ore) is best described as borehole drilling.