Earth Tides

The image above shows 24 hours of groundwater level data at a USGS monitoring site. These water levels are very much like a tidal signal, but this particular monitoring site isn't located anywhere near the ocean, and is instead at a site in the Appalachian Mountains!

This water level signal is caused not by ocean tides, but by Earth Tides.

Earth tides are much like ocean tides, except instead of the surface of the ocean rising and falling due to the gravitational influence of the Moon and Sun, it effects the solid Earth itself. In some wells, water levels will rise and fall in response to earth tides.