DEM Map

http://images.google.com/imgres?imgurl=http://nsidc.org/data/docs/daac/nsidc03t 04_0305_glas_dems/GLAS_Antarctic_DEM.jpg&imgrefurl=http://nsidc.org/data/docs/daac/nsidc0304_0305_glas_dems.gd.html&usg=__yO4g7JweSep_AS57rbNveHnMoV8=&h=300&w=360&sz=22&hl=en&start=7&tbnid=3lkRXeAj_dTT8M:&tbnh=101&tbnw=121&prev=/images%3Fq%3DDEM%26gbv%3D2%26hl%3Den%26sa%3DG

A DEM map is a digital elevation map that is a digital representation of terrain. The above picture is an example of a DEM map of Antarctica. I do not understand what the hole in the center is.