  Recent gravitational-wave observations from the LIGO and Virgo observatories
have brought a sense of great excitement to scientists and citizens the world
over. Since September 2015,10 binary black hole coalescences and one binary
neutron star coalescence have been observed. They have provided remarkable,
revolutionary insight into the "gravitational Universe" and have greatly
extended the field of multi-messenger astronomy. At present, Advanced LIGO can
see binary black hole coalescences out to redshift 0.6 and binary neutron star
coalescences to redshift 0.05. This probes only a very small fraction of the
volume of the observable Universe. However, current technologies can be
extended to construct "$3^\mathrm{rd}$ Generation" (3G) gravitational-wave
observatories that would extend our reach to the very edge of the observable
Universe. The event rates over such a large volume would be in the hundreds of
thousands per year (i.e.tens per hour). Such 3G detectors would have a 10-fold
improvement in strain sensitivity over the current generation of instruments,
yielding signal-to-noise ratios of 1000 for events like those already seen.
Several concepts are being studied for which engineering studies and reliable
cost estimates will be developed in the next 5 years.
