The present invention is directed to a customizable, modular foundation system using blocks and post-tensioning for improved structural integrity of the foundation. In some embodiments, the improved foundation system is provided by post-tensioning a plurality of blocks together such that cables or bars used for tensioning run in different directions than any of the joint lines between adjacent blocks. In this way, there are no bendable joints between adjacent blocks. Moreover, in some embodiments the blocks are hexagonal, such that each block has six sides and post-tensioning cables can run in at least three different directions. Structures or apparatuses may be secured to the foundation comprised of multiple blocks, with the foundation being rigid, customizable, and both assembled and usable in any desired location.