Stars are expected to undergo a pair instability supernovae (PISN) if they have helium cores larger than ~60Msun. These stars are expected to be fully disrupted due to encountering the pair instability region. Here, gamma rays are converted to electron-positrons and reduce the pressure support inside the star leading to a collapse followed by a shock which disrupts the entire star.  However, stars of slightly lower mass will undergo a series of pulses, rather than complete disruption when they encounter this instability, and produce a black hole remnant. Thus the boundary between these two outcomes, complete disruption or a black hole, determines the lower edge of the second black hole mass gap, a black hole mass region explorable by LIGO/VIRGO.  Even more massive stars may also not be fully disrupted by the PISN, and thus potentially form ~140Msun black holes at the upper edge of the second black hole mass gap. Here, I will discuss what conditions are necessary for a star to undergo a PISN, and what sets both the lower and upper boundaries of the second black hole mass gap. I will also discuss how robust the boundary between the different potential fates is, given uncertainties in the physics of stellar modelling and what this might entail for LIGO/VIRGO detections.