  After a quarter century since the detection of the last interstellar
carboxylic acid, acetic acid (CH$_3$COOH), we report the discovery of a new
one, the cis-trans form of carbonic acid (HOCOOH), toward the Galactic Center
molecular cloud G+0.693-0.027. HOCOOH stands as the first interstellar molecule
containing three oxygen atoms and also the third carboxylic acid detected so
far in the interstellar medium. Albeit the limited available laboratory
measurements (up to 65 GHz), we have also identified several pairs of unblended
lines directly in the astronomical data (between 75-120 GHz), which allowed us
to slightly improve the set of spectroscopic constants. We derive a column
density for cis-trans HOCOOH of $N$ = (6.4 $\pm$ 0.4) $\times$ 10$^{12}$
cm$^{-2}$, which yields an abundance with respect to molecular H$_2$ of 4.7
$\times$ 10$^{-11}$. Meanwhile, the extremely low dipole moment (about fifteen
times lower) of the lower-energy conformer, cis-cis HOCOOH, precludes its
detection. We obtain an upper limit to its abundance with respect to H$_2$ of
$\leq$ 1.2 $\times$10$^{-9}$, which suggests that cis-cis HOCOOH might be
fairly abundant in interstellar space, although it is nearly undetectable by
radio astronomical observations. We derive a cis-cis/cis-trans ratio $\leq$ 25,
consistent with the smaller energy difference between both conformers compared
with the relative stability of trans- and cis-formic acid (HCOOH). Finally, we
compare the abundance of these acids in different astronomical environments,
further suggesting a relationship between the chemical content found in the
interstellar medium and the chemical composition of the minor bodies of the
Solar System, which could be inherited during the star formation process.
