The monthly land mass grids contain terrestrial water masses (aquifers, river basins, etc.) given as equivalent water thickness derived from GRACE gravity data, in GeoTiff format. Equivalent water thickness is used to represent changes in water mass as height changes since the area of a grid pixel remains only the height can change as the amount of water changes. The time coverage for the monthly grids are determined by GRACE months. For the list of GRACE month dates visit http://grace.jpl.nasa.gov/data/grace-months/. For information please visit http://grace.jpl.nasa.gov/data/get-data/monthly-mass-grids-land/.