Effervescent Spikes in M-theory

AdS$_3 \times$ S$^3 \times$ S$^3$ solutions warped over a Riemann surface, $\Sigma$, are indexed by a parameter, $\gamma$, that defines the superconformal algebra, $D(2,1; \gamma) \oplus D(2,1; \gamma)$ they preserve. We show that these solutions come from multiple back-reacted M2-M5 spikes, and that different values of $\gamma$ correspond to different scaling limits of the same M2-M5 solutions. We find that when $\gamma$ switches from positive to negative, the infrared region of the AdS$_3$ switches from the tip of spikes, far from the M5 branes, to the bottom of the spikes, far from the M2 branes. We also explain how the bubbling negative-$\gamma$ solutions emerge from the geometric transition of multiple M2-M5 spikes.