Abstract: A rare opportunity to explore the fate of a three-dimensional gas of highly mobile electrons confined to their lowest Landau levels is provided by elemental semi-metals. Coulomb interaction, neglected in the band picture, is expected to become significant in this extreme quantum limit, with poorly understood consequences. Our study of the angular-dependent Nernst effect in bismuth finds, on top of a complex single-particle spectrum, collective field scales of unidentified origin. Each time a Landau level crosses the Fermi level, the Nernst response sharply peaks. All such peaks are resolved by the experiment, and their complex angular dependence is in very good agreement with the theory. Beyond the quantum limit, we resolve additional Nernst peaks signaling a cascade of Landau sublevels caused by electron interaction. The collective excitaions generating the additional sublevels break the rotational symmetry of the crystal.