Oh, and let me add, that I would not consider this an "underground" piece of prog. A solo album by any member of Genesis at the time, could not be underground. They were pretty mainstream at the time.
After all, in the same year, there were a great number of absolute masterpieces of prog (much better than Smallcreep's Day, IMO), that were substantially more underground.
Sure, they were much more on the avant-prog end of the spectrum, and more edgy, thorny, atonal, etc., but also incredible. And substantially more underground.
Eskaton - 4 Visions (Magma influenced)
Present - Triskaidekaphobie (heavy, Crimson-ish, avant-garde)
Art Zoyd -
Generation Sans Futur (dark, atonal, avant-garde)
Picchio Dal Pozzo - Abbiamo Tutti I Suoi Problemi (jazzy Canterbury influenced)
Abus Dangereux - Le Quatrieme Mouvement (prog-fusion, with Magma influneces)