  The Landauer principle states that at least $k_B T \ln 2$ of energy is
required to erase a 1-bit memory, with $k_B T$ the thermal energy of the
system. We study the effects of inertia on this bound using as one-bit memory
an underdamped micro-mechanical oscillator confined in a double-well potential
created by a feedback loop. The potential barrier is precisely tunable in the
few $k_B T$ range. We measure, within the stochastic thermodynamic framework,
the work and the heat of the erasure protocol. We demonstrate experimentally
and theoretically that, in this underdamped system, the Landauer bound is
reached with a 1 % uncertainty, with protocols as short as 100 ms.
