  In spite of its rather weird properties which include violation of the
dominant-energy condition, the requirement of superluminal sound speed and
increasing vacuum-energy density, phantom energy has recently attracted a lot
of scientific and popular interests. In this letter it is shown that in the
framework of a general k-essence model, vacuum-phantom energy leads to a
cosmological scenario having negative sound speed and a big-rip singularity,
where the field potential also blows up, which might occur at an almost
arbitrarily near time in the future that can still be comfortably accommodated
within current observational constraints.
