  The first direct detection of gravitational waves emitted from a pair of
merging black holes in 2015 has been heralded as one of most significant
scientific breakthroughs in physics and astronomy of the 21st century.
Motivated by the tremendous scientific opportunities now opened by
gravitational-wave observatories and recognizing that to fully exploit the new
field will require new observatories that may take 15 to 20 years from
conception until operations begin, the Gravitational Wave International
Committee (GWIC) convened a subcommittee to examine the path to build and
operate a network of future ground-based observatories, capable of extending
the observational GW horizon well beyond that currently attainable with the
current generation of detectors.
  This report is the first in a six part series of reports by the GWIC 3G
Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the
Edge of the Universe (this report), ii) The Next Generation Global
Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next
Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave
Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based
Gravitational-wave Observatories: Synergies with Other Scientific Communities,
and vi) An Exploration of Possible Governance Models for the Future Global
Gravitational-Wave Observatory Network.
