  Robotic exploration of underground environments is a particularly challenging
problem due to communication, endurance, and traversability constraints which
necessitate high degrees of autonomy and agility. These challenges are further
exacerbated by the need to minimize human intervention for practical
applications. While legged robots have the ability to traverse extremely
challenging terrain, they also engender new challenges for planning,
estimation, and control. In this work, we describe a fully autonomous system
for multi-robot mine exploration and mapping using legged quadrupeds, as well
as a distributed database mesh networking system for reporting data. In
addition, we show results from the DARPA Subterranean Challenge (SubT) Tunnel
Circuit demonstrating localization of artifacts after traversals of hundreds of
meters. These experiments describe fully autonomous exploration of an unknown
Global Navigation Satellite System (GNSS)-denied environment undertaken by
legged robots.
