Undermesh means that the base, in this case, was built below the ground, in an area that isn't supposed to be playable. However, since there are probably about 50-60 holes in the mesh on the Center alone, or ways to get under the playable map area, people frequently, and commonly, have bases down there. 45-60x60x30 are the dimensions of the base in question, meaning 45-60 foundations long, by 60 foundations wide, by 30 walls high.