The Bouguer gravity anomaly means that gravity is not evenly distributed

I just learned about the Bouguer gravity anomaly, which the US Geological Survey defines as:

Measurements of the gravitational field vary slightly from place to place due to the composition and structure of Earth's crust. This digital grid describes the complete Bouguer gravity anomaly for the conterminous US.

For example, here's a map of the gravitational pull in Illinois, Indiana, and Ohio:

The colors on this complete Bouguer gravity anomaly map (Bankey and others, 1995) represent gravity variations resulting from lateral density variations in the Earth. The more positive anomalies (red colors) occur in areas with average density greater than the Bouguer reduction density of 2.67 gm/cc, whereas the more negative anomalies (blue colors) occur in areas of lower density. The color scale used to create this image is nonlinear to best represent the range of values.

Gravitational anomalies are not just limited to those 3 states, of course; they're found all around the world (although I don't think anyone is particularly surprised that Ohio is an astrophysical phenomenon in-and-of itself).

A cursory glimpse at Wikipedia informs me that there are other kinds of gravitational anomalies besides the Bouguer, but I'm not smart enough to summarize and synthesize that information for you.

Bouguer gravity anomaly grid for the conterminous US [USGS]

Gravity is not uniform. It varies geographically. [Frank Jacobs / Big Think]

Image: Public Domain via NASA