Individual four-sided shaped modules used in an assembly for underground storage of storm water and other fluid storage needs. Modules are assembled into a resultant four-sided tiling shape for maximized structural strength and material use efficiency. Internal four-sided shaped modules are assembled and encased by external four-sided shaped modules. Internal adjacent modules are in direct fluid communications with one another through a channel-less chamber. Internal four-sided shaped modules drain into four-sided shaped modules chamber where fluid is either stored or drained. Assemblies include various top and side pieces along with access ports for entry into said assembly.