A container has a pair of opposing heads, and a wall section between the heads has a square cross-section and planar side walls joined together by rounded corners. The side walls tend to deflect outwardly while under pressure, but are supported externally by a support system such as the walls of a cargo hold. Another container comprises an outside tank and a flexible membrane tank inside the outside tank, an annular space being defined in between, where a first fluid is charged into the membrane tank and a second fluid is charged into the annular space in order to discharge the first fluid. In another embodiment, a long tube having a square cross-section is coiled in a support structure and made gas-tight for holding a compressed fluid, and adjacent coils and the support structure limit expansion of the tube.