Field3D is a library for storing voxel data. It provides C++ classes that handle in-memory storage and a file format based on HDF5. The library and file format is both flexible and extendable and supports heterogeneous storage of data structures, bit depths, transformations/mappings, and metadata in a single file. A plugin and class factory architecture allows new data structures, mappings, and file I/O routines to be added. Dense and MAC-fields handle basic fluid simulation tasks and volume rendering, and optimization of used/unused space is provided by sparse fields, which include an out-of-core/dynamic loading mechanism. Procedural fields are also supported.