ESRI ArcInfo Grid
ESRI ArcInfo Grid (or ESRI Grid, ArcGrid; "ArcInfo" may also be spelled Arc/Info) is a geospatial raster/grid format. There is a binary version (ADF), and an "ASCII" version of the format.
An ADF "file" is actually a directory containing multiple files (most with an .adf extension) that don't all use the same format. Some of the files start with bytes
00 00 27 0A FF FF.