Freshened water exists in continental shelf aquifers worldwide. One remarkable example is the US Atlantic shelf, where groundwater in shallow sand aquifers >100 km offshore has salinity that is 10 % of seawater salinity. Expedition 501 will drill sites off Massachusetts, US, to estimate how much freshened water there is, processes that emplace the water, and rates of emplacement. The expedition will also assess whether salinity patterns have important implications for microbial processes.