Recent studies indicate that groundwater may contribute significant fluxes of dissolved chemical species to the oceans. The magnitude of such fluxes is influenced by biogeochemical processes occurring in the subterranean estuary, defined as the mixing zone between groundwater and seawater in a coastal aquifer. In contrast to surface estuaries, little is known about chemical reactions in subterranean estuaries mainly because they are difficult to sample due to their subsurface location. Here we report the discovery of an "Iron Curtain" in a subterranean estuary on Cape Cod, MA, USA. The term "Iron Curtain" refers to the precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands at the groundwater-seawater interface. The formation of an Iron Curtain is not likely limited to the study area of interest; any coastal aquifer bearing high concentrations of dissolved ferrous iron that intercepts surface water is likely to exhibit this feature. As naturally-occurring iron oxides are strong adsorbers and concentrators of many dissolved chemical species, the occurrence of an Iron Curtain has broad implications for transport of natural and anthropogenic materials from aquifers into coastal waters.