  We discuss the capability of a third-generation ground-based detector such as
the Einstein Telescope (ET) to enhance our astrophysical knowledge through
detections of gravitational waves emitted by binaries including
intermediate-mass and massive black holes. The design target for such
instruments calls for improved sensitivity at low frequencies, specifically in
the ~ 1-10 Hz range. This will allow the detection of gravitational waves
generated in binary systems containing black holes of intermediate mass, ~
100-1000 solar masses. We primarily discuss two different source types --
mergers between two intermediate mass black holes (IMBHs) of comparable mass,
and intermediate-mass-ratio inspirals (IMRIs) of smaller compact objects with
mass ~ 1-10 solar masses into IMBHs. IMBHs may form via two channels: (i) in
dark matter halos at high redshift through direct collapse or the collapse of
very massive metal-poor Population III stars, or (ii) via runaway stellar
collisions in globular clusters. In this paper, we will discuss both formation
channels, and both classes of merger in each case. We review existing rate
estimates where these exist in the literature, and provide some new
calculations for the approximate numbers of events that will be seen by a
detector like the Einstein Telescope. These results indicate that the ET may
see a few to a few thousand comparable-mass IMBH mergers and as many as several
hundred IMRI events per year. These observations will significantly enhance our
understanding of galactic black-hole growth, of the existence and properties of
IMBHs and of the astrophysics of globular clusters. We finish our review with a
discussion of some more speculative sources of gravitational waves for the ET,
including hypermassive white dwarfs and eccentric stellar-mass compact-object
binaries.
