"Mirrors That Hang on Glass Threads" is the first video by the Advanced LIGO Documentary Project, under its two-year grant from the National Science Foundation to film the new era of gravitational-wave astronomy. The era was launched by the historic detection of gravitational waves in September 2015, opening up the 95% of the universe that has been dark to our existing observatories and satellites. The video describes one of the brilliant, exquisite innovations in the 40-year search for gravitational waves - one that led to the detection of ripples in space time caused by the collision of two black holes 1.3 billion years ago. Director Les Guthman filmed the inside story of the discovery during the first year of the Advanced LIGO Documentary Project. His feature documentary, "LIGO," will be released in fall 2017. The second documentary will not finish principal photography until summer 2018.