Recent underwater explorations have revealed unique hot spots of biogeochemi­cal activity at several submerged ground­water vents in Lake Huron, the third larg­est of the Laurentian Great Lakes. Fueled by venting groundwater containing high sul­fate and low dissolved oxygen, these under­water ecosystems are characterized by sharp physical and chemical gradients and spec­tacularly colorful benthic mats that overlie carbon-rich sediments. Here, typical lake inhabitants such as fish and phytoplankton are replaced by communities dominated by microorganisms: bacteria and archaea that perform unique ecosystem functions. Shal­low, sunlit sinkholes are dominated by pho­tosynthetic microorganisms and processes, while food webs in deep aphotic sinkholes are supported primarily by chemosynthesis.