The origins of the stellar-mass black hole mergers discovered by LIGO/Virgo are still unknown. In an article published on November 1, 2019 in Physical Review Letters former Columbia University Lecturer Imre Bartos (now at the University of Florida), as well as current faculty including Profs. Zoltan Haiman, Szabi Marka and Zsuzsa Marka, have shown that accretion disks of Active Galactic Nuclei (AGNs) promote the mergers of their captive black holes, and the majority of black holes within the AGN disks will undergo hierarchical mergers---with one of the black holes being the remnant of a previous merger. 40% of AGN-assisted mergers detected by LIGO/Virgo will include a black hole with mass in excess of 50 Msun, the mass limit from stellar core collapse. These results suggest that LIGO's heaviest merger so far, GW170729, could have originated from this channel. The article was selected as an Editors' Suggestion and as a "Featured in Physics" focus article.