Efimov’s scenario: The energy of three identical bosons is plotted as a function of the inverse two-body scattering length 1/a. Three different regions exist, the three-atom continuum for E>0 (gray), the atom-dimer continuum for a>0 and -ħ2/(ma2)<E<0 (blue), and the trimer region (green). The Efimov states are shown by the green solid lines. They emerge at the three-atom threshold for a<0 and connect to the atom-dimer threshold for a>0. (To illustrate the series of Efimov states, we have artificially reduced the universal scaling factor from 22.7 to 2.) The arrows indicate situations where Efimov states couple to the thresholds, leading to pronounced resonance phenomena.