  Recent observations and stellar dynamics simulations suggest that thousand
solar mass black holes can form in compact massive young star clusters. Any
such clusters in the bulge of their host galaxy will spiral to the center
within a few hundred million years, where their intermediate-mass black holes
are likely to merge eventually with the galaxy's supermassive black hole. If
such mergers are common, then future space-based gravitational wave detectors
such as the Laser Interferometer Space Antenna will detect them with such a
high signal to noise ratio that towards the end of the inspiral the orbits will
be visible in a simple power density spectrum, without the need for matched
filtering. We discuss the astrophysics of the inspiral of clusters in the
nuclear region of a galaxy and the subsequent merger of intermediate-mass with
supermassive black holes. We also examine the prospects for understanding the
spacetime geometry of rotating black holes, based on phase connection of the
strong signals visible near the end of these extreme mass ratio inspirals.
