A gigantic freshwater aquifer is hiding under the salty Atlantic Ocean, just off the northeastern coast of the United States, a new study finds.

While the aquifer’s exact size is still a mystery, it may be the largest of its kind, taking up a region stretching from at least Massachusetts to southern New Jersey, or nearly 220 miles (350 kilometers). The area includes the coastlines of New York, Connecticut and Rhode Island. This aquifer may contain about 670 cubic miles (2,800 cubic kilometers) of slightly salty water (we’ll explain its slight saltiness later).

This water isn’t young, either. The researchers said they suspect that much of it is from the last ice age.

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