Ubiquitous carbonic acid, H₂CO₃, a key molecule in biochemistry, geochemistry, and also extraterrestrial chemistry, is synthetically known [1], also from spectroscopic studies [2], but it is often considered, even up to the present day, a somewhat mysterious “non-existing” molecule. In fact, the molecule has never been directly seen because high pressure is needed to stabilize it, as easily shown by electronic-structure theory. After an eight-years research study, the crystal structure of carbonic acid has been determined from neutron-diffraction data [3] on a deuterated sample in a specially built hybrid clamped cell [4] made  from “Russian alloy”. At 1.85 GPa, D₂CO₃ crystallizes in the monoclinic space group P2₁/c with a = 5.392(2), b = 6.661(4), c = 5.690(1) Å, β = 92.66(3)°, Z = 4, with one symmetry-inequivalent anti-anti shaped D₂CO₃ molecule forming dimers [5], as qualitatively predicted before. Quantum chemistry from plane waves using local orbitals evidences π bonding within the CO₃ molecular core, very strong hydrogen bonding between the molecules, and a massive influence of the Madelung field; phonon calculations emphasize the locality of the vibrations, being rather insensitive to the extended structure. Now that carbonic acid has been firmly established, this may be useful for other fields, for example CO₂ “sequestration” and its consequences. Likewise, carbonic acid probably plays a role in our solar system, say, on outer gas planets such as Uranus or Neptune and, also, on the Jupiter moon Europa. Finally, many chemistry textbooks must be rewritten because the simplest molecule consisting of water and carbon dioxide actually exists and can be observed.