Significance Quantum states of molecular vibrational polaritons, hybrid half-light, half-matter quasi-particles, are studied using ultrafast coherent two-dimensional infrared spectroscopy. Valuable physical insights such as existence of hidden dark states and ultrafast interactions between dark states and bright polariton states are unambiguously revealed. These results not only advance coherent two-dimensional spectroscopy, but also have significant implications in harvesting the creation of molecular vibrational polaritons for infrared photonic devices, lasing, molecular quantum simulation, as well as chemistry by tailoring potential energy landscapes.