What Was Not to Be: The Apollo 18, 19 and 20 Landing Sites

From Footprints in the Dust, chapter 11, by Colin Burgess, pg. 336:

Apollo 18’s Lunar module was scheduled to land in Schroter’s Valley, the site of intriguing transient lunar phenomena and possibly even volcanic activity. The two-man landing crew of Apollo 19 would then have explored the collapsed lava tubes of Hyginus Rille. The most hazardous but ultimately benficial mission of all would have