When two black holes merge, their event horizons touch first. For a fraction of a second, a single giant black hole contains two distinct singularities on an inescapable collision course.
To understand how this happens, you have to separate the "black hole" from the "singularity." The event horizon is simply the boundary where gravity becomes so strong that light cannot escape. The singularity is the actual physical object at the center—a region where, according to classical general relativity, matter is crushed to infinite density and spacetime curvature becomes infinite.
As the merging event horizons connect, they form a single, rapidly shifting, peanut-shaped boundary that quickly settles into a sphere.
Inside an event horizon, space and time behave differently. Space itself flows inward toward the center faster than the speed of light. Because all physical paths inside a black hole lead inexorably inward, the two singularities cannot orbit each other indefinitely or remain apart. They are dragged together and collide, merging into a single, more massive singularity. If the original black holes were spinning—which they almost always are—the newly combined singularity will not be a point, but a rapidly rotating ring, known as a ring singularity.
This new singularity does not contain the exact combined mass of the original two. In the final moments before they merge, the black holes whip around each other at a significant fraction of the speed of light, churning the fabric of spacetime. This violent acceleration converts a portion of the black holes' mass into pure energy, which radiates outward across the universe as gravitational waves. When the LIGO observatory detected the first black hole merger in 2015, the resulting singularity was three solar masses lighter than the two original black holes combined. Three entire suns' worth of mass had been transformed into gravitational ripples in a fraction of a second.
There is one major caveat to this process: quantum physics. Most physicists believe that true singularities—points of literally infinite density—do not actually exist. The concept of a singularity is likely just a placeholder indicating where current mathematical models of gravity break down. If modern theories like string theory or loop quantum gravity are correct, the centers of black holes might actually be hyper-dense quantum states, such as fuzzballs or Planck stars. If so, those two exotic objects simply smash together to form a larger, immensely dense core.