The Free On-line Dictionary of Computing (30 December 2018):

brontobyte

    A proposed unit of data equal to 10^27 bytes.  A
   brontobyte is 1000^9 bytes or 1000 yottabytes.

   "Bronto-" is not an official prefix and the term brontobyte is
   generally attributed to the IBM Dictionary of Computing.

   One brontobyte would be enough data to store a three-dimensional
   map of the Earth with one byte for each voxel of a
   one-centimetre grid.

   See prefix.

   [Where did IBM get it from?]

   (2013-11-04)