Pair instabilities in supernovae might prevent the formation of black holes with masses between \mathchar 21016 50 M _ { \odot } and \mathchar 21016 130 M _ { \odot } .
Multiple generations of black-hole mergers provide a possible way to populate this ‘ ‘ mass gap ’ ’ from below .
However this requires an astrophysical environment with a sufficiently large escape speed to retain merger remnants , and prevent them from being ejected by gravitational-wave recoils .
We show that , if the mass gap is indeed populated by multiple mergers , the observation of a single black-hole binary component in the mass gap implies that its progenitors grew in an environment with escape speed v _ { esc } \gtrsim 50 km/s .
This is larger than the escape speeds of most globular clusters , requiring denser and heavier environments such as nuclear star clusters or disks-assisted migration in galactic nuclei .
A single detection in the upper mass gap would hint at the existence of a much larger population of first-generation events from the same environment , thus providing a tool to disentangle the contribution of different formation channels to the observed merger rate .