It is explained in the passage that under the hard surface of the earth there ____ .

When it was formed many million years ago the earth
was a liquid. It is still cooling and many miles below
the hard crust is still hot. However, in some places the
heat is closer to the surface. These places are
associated with volcanic activity or hot sulphur
springs. By drilling deep into the earth's crust we can
reach rocks that are much warmer than those at the
surface. Pumping water down into contact with these
rocks and extracting the steam so produced is a
source of energy that can be used to produce
electricity. It is called geothermal energy.