Notable underground film events of 1975 were the formation of Berks Filmmakers, the debut of the Rocky Horror Picture Show and Amos Poe making Night Lunch.
Notable underground film events of 1973 were P. Adams Sitney publishing Visionary Film, John Waters made Female Trouble and David Lynch directed The Amputee.
Notable underground film events for 1973 were a widespread appreciation for Stan Brakhage developed; Alejandro Jodorowsky made The Holy Mountain and more.
Kill Your Timid Notion is a festival of underground film and performances, including work by Ken Jacobs, Kurt Kren, Andrew Lampert, Hollis Frampton and more.
Notable underground films in 1972 were John Waters’ Pink Flamingos, Robert Downey Sr.’s Greaser’s Palace; plus Kenneth Anger published his own obituary.
Some notable underground film events in 1971 were Jonas Mekas ending his Movie Journal column, Willard Maas passing away and the first festival was held.
Visionary Film by P. Adams Sitney is still a must-read history of underground film even though it basically says that history petered out in the early 70s.
“Underground Film, Into the Light: Two Sides of the Projected Image in American Art, 1945-1975” by Bruce Kashmere is a nice little overview of the American avant garde film movement, mostly about the merging of filmmaking and traditional art. There’s some coverage of Maya Deren, Jonas Mekas (always nice) and Michael Snow
In Switzerland, you can watch the freakiest films in the world at the Lausanne Underground Film Festival, including ones by Crispin Glover and Jonas Mekas.