The Gruithuisen domes are three volcanic domes that lie on the western rim of Mare Imbrium (the Sea of Storms). Unlike most volcanic materials on the Moon, the three domes are relatively low in the metallic elements iron and titanium, and relatively high in silicon. This composition yields an unusually viscous (sticky) lava that might be due to fractional crystallization at the end of eruptive life; or a result of remelting of high-silica crust at the beginning of eruptive life. So are the domes some of the oldest volcanic deposits on the Moon - or some of the youngest? Heimdall will help us find out.