A general purpose library and file format for storing scientific data
http://hdf.ncsa.uiuc.edu/hdf4.html

HDF is a general purpose library and file format for storing scientific data.
HDF can store two primary objects: datasets and groups. A dataset is 
essentially a multidimensional array of data elements, and a group is a 
structure for organizing objects in an HDF file. Using these two basic 
objects, one can create and store almost any kind of scientific data 
structure, such as images, arrays of vectors, and structured and unstructured 
grids. You can also mix and match them in HDF files according to your needs.