Layer by layer, artifact by artifact — an archaeologist reconstructs the history of an ancient settlement. Similarly, astrophysicists sifting through 259 records from LIGO–Virgo–KAGRA saw a clear layered structure among merging black holes. Each layer represents hundreds of millions of years of evolution, captured in gravitational waves.

The lowest and most powerful layer: lightweight holes — about 70% of all finds. Their masses cluster near 10 solar masses, both holes are nearly equal, and spin axes align with the orbit. This is the typical product of isolated stellar evolution of massive stars, where the finale is determined by special supernovas. But at about 14 solar masses — a sharp dip. Finds there are 26 times rarer than expected from a smooth distribution. Like a cultural layer scorched by fire.

Above are two intermediate horizons. The first, horizontal: unequal pairs — the secondary hole is stuck around 10 M⊙, while the primary climbs to 40. Spins are mixed, and the event rate drops steeply with mass — possible traces of rare captures in binary star systems. The second, diagonal: near-twins with a peak at 31 M⊙. Their effective spins are small, but spin axes are likely chaotically tilted. Such a signature is expected from pairs forged in the tight confines of globular clusters.

The topmost layer begins above 47 solar masses. Holes here are massive, mass ratios vary widely, and spins are large and randomly oriented. This is a clear sign of hierarchical mergers: holes grow generation after generation by colliding in dense environments. The archaeological analogy is complete — village remains compressed into a mound, on which a city was later built.

This stratigraphy turns every observation into an archaeological fact. Future LIGO gravitational wave telescopes, such as the Einstein Telescope, will multiply the number of finds tenfold, and allow the population to be layered even more finely, tracing the history of black holes from near the birth of stars to the present day.