Ocean Floor
The youngest parts of the ocean floor are hot and shallow compared to older parts that are cold and deep.
Young is Hot, Old is Cold
Heat flow is greatest at very young ocean floor, because it starts out as molten lava, then cools with age. In the map above, red areas are hotter, greener areas are cooler.
Young is Shallow, Old is Deep
As ocean lithosphere ages and cools over time, the cooling causes it to become denser (like most substances that contract when cooled), and thus subside to greater depths. The mass of rock from the deep mantle to the surface is roughly constant wherever you go, but because the rock density depends on temperature, cooler places are not as high, and are thus covered with deeper water. Younger areas with high heat flow are shallower.