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 been Apollo 20. The Lunar Module was to have been upgraded to allow the two man crew a stay of up to six days on the lunar surface, enabling them to visit and excavate the floor of the immense Copernicus Crater.