  The third generation of gravitational wave observatories, aiming to provide
100 times better sensitivity than currently operating interferometers, is
expected to establish the evolving field of gravitational wave astronomy. A key
element for achieving the ambitious sensitivity goal is the exploration of new
interferometer geometries, topologies and configurations. In this article we
review the current status of the ongoing design work for third-generation
gravitational wave observatories. The main focus is set on the evaluation of
the detector geometry and detector topology. In addition we discuss some
promising detector configurations and potential noise reduction schemes.
