Gravity more or less points down, as you know, but there are very subtle variations in gravity which are related to things like mountains and valleys and changes in rock. If you fly a satellite in space and measure gravity very carefully, you can start to map out changes in gravity and make inferences about the structures which cause those changes. In the end, David Sandwell and colleagues have used these techniques to make amazingly detailed maps of the seafloor, like this one . David also has an SGI and time to play, and has made some cool closeups . Here are some basic Earth facts which you will need to know for this class.
Earth is differentiated.
Differentiation happened when heat from impacts and radioactivity caused part of Earth to melt, probably during or shortly after Earth's formation. Dense material sank to center, light material floated to surface. Structure of Earth is inferred from many sources, including seismology and gravity.