The occurrence of a bubble nuclei induced by an inversion of s and d proton states is investigated. First, we analyze the origin of this quantum effect in the calcium isotopes. Pairing effects which can play against the bubble formation are evaluated. Then, we discuss the experimental data which favor the bubble hypothesis. This pleads for a measurement of the charge density of $^{46}$Ar in the forthcoming years, with the advent of electron scattering experiments in next generation exotic beam facilities such as FAIR or RIBF.