Millennial magma reserves Lava erupts after being stored as magma deeper underground. Silica-rich magmas characteristic of volcanoes like Pinatubo are stored for thousands to hundreds of thousands of years. However, we have little understanding of the time scales for deeper basaltic magma like that supplying Icelandic volcanoes. Mutch et al. used diffusion of chromium and aluminum atoms to show that these magmas are stored for hundreds to thousands of years at the crust-mantle boundary. This discovery elucidates the time scale for understanding how magma is created and stored and how it erupts. Science , this issue p. 260