  In a search for eclipsing white dwarfs using the Zwicky Transient Facility
lightcurves, we identified a deep eclipsing white dwarf with a dark, substellar
companion. The lack of an infrared excess and an orbital period of 10 hours
made this a potential exoplanet candidate. We obtained high-speed photometry
and radial velocity measurements to characterize the system. The white dwarf
has a mass of $0.50\pm0.02\,\mathrm{M_{\odot}}$ and a temperature of
$10900\pm200\,$K. The companion has a mass of
$0.059\pm0.004\,\mathrm{M_{\odot}}$ and a small radius of
$0.0783\pm0.0013\,\mathrm{R_{\odot}}$. It is one of the smallest transiting
brown dwarfs known and likely old, $\gtrsim 8\,$Gyr. The ZTF discovery
efficiency of substellar objects transiting white dwarfs is limited by the
number of epochs and as ZTF continues to collect data we expect to find more of
these systems. This will allow us to measure period and mass distributions and
allows us to understand the formation channels of white dwarfs with substellar
companions.
