  The thermodynamic uncertainty relation expresses a seemingly universal
trade-off between the cost for driving an autonomous system and precision in
any output observable. It has so far been proven for discrete systems and for
overdamped Brownian motion. Its validity for the more general class of
underdamped Brownian motion, where inertia is relevant, was conjectured based
on numerical evidence. We now disprove this conjecture by constructing a
counterexample. Its design is inspired by a classical pendulum clock, which
uses an escapement to couple the motion of an oscillator to another degree of
freedom (a "hand") driven by an external force. Considering a thermodynamically
consistent, discrete model for an escapement mechanism, we first show that the
oscillations of an underdamped harmonic oscillator in thermal equilibrium are
sufficient to break the thermodynamic uncertainty relation. We then show that
this is also the case in simulations of a fully continuous underdamped system
with a potential landscape that mimics an escaped pendulum.
