  This paper discusses the nanopores emerging and developing in a liquid
dielectric under the action of the ponderomotive electrostrictive forces in a
nonuniform electric field. It is shown that the gradient of the electric field
in the vicinity of the rupture (cavitation nanopore) substantially increases
and determines whether the rupture grows or collapses. The cavitation rupture
in the liquid (nanopore) tends to stretch along the lines of the original
field. The mechanism of the breakdown associated with the generation of
secondary ruptures in the vicinity of the poles of the nanopore is proposed.
The estimations of the extension time for nanopore in water and oil (polar and
nonpolar liquids, respectively) are presented. A new mechanism of nano- and
subnanosecond breakdown in the insulating (transformer) oil that can be
realized in the vicinity of water microdroplets in modern nanosecond
high-voltage devices is considered
