Sinking slabs of ocean crust and rising plumes of hot rock in Earth's mantle are observed to behave differently below one megameter (1,000 kilometers) depth. Two explanations for this behavior were published on Dec. 11, 2015. At left, Rudolph et al. (Science, 2015) propose a viscosity increase (dark blue) below the megameter boundary. At right, Ballmer et al. (Science Advances, 2015) propose a density increase due to accumulated ocean crust (dark squiggles) below the boundary.