  We present the discovery of a vast cloud of ionized gas 13$^{\prime}$ (32
kpc) north of the interacting system M51. We detected this cloud via deep
narrow-band imaging with the Burrell Schmidt Telescope, where it appears as an
extended, diffuse H$\alpha$-emitting feature with no embedded compact regions.
The Cloud spans $\sim$10$^{\prime}\times$3$^{\prime}$ (25$\times$7.5 kpc) in
size and has no stellar counterpart; comparisons with our previous deep
broadband imaging show no detected continuum light to a limit of $\mu_{\rm lim,
B} \sim$30 mag arcsec$^{-2}$. WIYN SparsePak observations confirm the cloud's
kinematic association with M51, and the high NII/H$\alpha$, SII/H$\alpha$, and
OI/H$\alpha$ line ratios we measure imply a hard ionization source such as AGN
photoionization or shock heating rather than photoionization due to young
stars. Given the strong NII emission, we infer roughly solar metallicity for
the cloud, ruling out an origin due to infall of primordial gas. Instead we
favor models where the gas has been expelled from the inner regions of the M51
system due to tidal stripping or starburst/AGN winds and has been subsequently
ionized either by shocks or a fading AGN. This latter scenario raises the
intriguing possibility that M51 may be the nearest example of an AGN fossil
nebula or light echo, akin to the famous "Hanny's Voorwerp" in the IC 2497
system.
