At the end of an Arctic winter, we found ponded water 500 m under a glacier. We explored the chemistry and microbiology of this unique, dark, and cold aquatic habitat to better understand ecology beneath glaciers. The water was occupied by cold-loving and cold-tolerant microbes with versatile metabolisms and broad habitat ranges and was depleted in compounds commonly used by microbes. These results show that microbes can become established beneath glaciers and deplete nutrients within months.