The ground beneath your feet may feel completely still.

But it isn't.

Twice a day, the solid Earth itself rises and falls by measurable amounts.

The reason is surprisingly familiar:

The Moon.

Just as the Moon's gravity helps create ocean tides, it also produces tides in the solid Earth.

The effect is called an Earth tide, or solid Earth tide.

As the Moon and Sun pull on our planet, the Earth's crust flexes slightly.

In some locations, the ground can move vertically by tens of centimeters over the course of a tidal cycle.

You don't feel it happening.

There is no dramatic shaking.

No sudden drop.

No visible wave sweeping across the landscape.

The movement is incredibly slow and gradual.

But sensitive instruments can measure it.