The
cloud
collapsed.
The
sky
darkened.
A
flood
of
superheated
gas,
ash,
and
rocks
crashed
down
the
sides
of
the
mountain
at
speeds
of
up
to
450
kilometres
an
hour.
This
first
surge
hit
the
settlements,
such
as
Herculaneum
and
Oplontis,
closest
to
the
crater.