Whether or not the Earth's mantle gets stiffer at great depths is an issue that has bitterly divided geophysicists. If it does, the lower mantle may be sealed off from the upper mantle, as the composition of some surface rocks implies; if not, the mantle may mix from top to bottom, as other indicators suggest. A new study settles the issue, showing to most geophysicists' satisfaction that the mantle does stiffen drastically with depth. But this very stiff lower mantle is allowing theorists to find a middle ground between whole-mantle and layered mixing.