  High-aspect-ratio mechanical resonators are pivotal in precision sensing,
from macroscopic gravitational wave detectors to nanoscale acoustics. However,
fabrication challenges and high computational costs have limited the
length-to-thickness ratio of these devices, leaving a largely unexplored regime
in nano-engineering. We present nanomechanical resonators that extend
centimeters in length yet retain nanometer thickness. We explore this expanded
design space using an optimization approach which judiciously employs fast
millimeter-scale simulations to steer the more computationally intensive
centimeter-scale design optimization. By employing delicate nanofabrication
techniques, our approach ensures high-yield realization, experimentally
confirming room-temperature quality factors close to theoretical predictions.
The synergy between nanofabrication, design optimization guided by machine
learning, and precision engineering opens a solid-state path to
room-temperature quality factors approaching 10 billion at kilohertz mechanical
frequencies -- comparable to the performance of leading cryogenic resonators
and levitated nanospheres, even under significantly less stringent temperature
and vacuum conditions.
