VMS Help FORTRAN, /CHECK, UNDERFLOW  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Enables or disables floating underflow exceptions.  If
  /CHECK=UNDERFLOW is specified, error messages are issued for the
  first two floating underflows.  Regardless of the setting of the
  UNDERFLOW qualifier, zero is stored as the result that underflowed
  and the program continues.