  Cool white dwarf stars are usually found to have an outer atmosphere that is
practically pure in hydrogen or helium. However, a small fraction have traces
of heavy elements that must originate from the accretion of extrinsic material,
most probably circumstellar matter. Upon examining thousands of Sloan Digital
Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS
J073842.56+183509.6 shows the most severe metal pollution ever seen in the
outermost layers of such stars. We present here a quantitative analysis of this
exciting star by combining high S/N follow-up spectroscopic and photometric
observations with model atmospheres and evolutionary models. We determine the
global structural properties of our target star, as well as the abundances of
the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca,
and Fe. The relative abundances of these elements imply that the source of the
accreted material has a composition similar to that of Bulk Earth. We also
report the signature of a circumstellar disk revealed through a large infrared
excess in JHK photometry. Combined with our inferred estimate of the mass of
the accreted material, this strongly suggests that we are witnessing the
remains of a tidally disrupted extrasolar body that was as large as Ceres.
