Seastalker is one of the classic infocom games. When they started intoducing their difficulty ratings, this was classed as one of their begginer series. Although not quite as mind-numbingly hard as some of the others in the series, it's great fun, and is quite challenging at points. All of the infocom games, with scans of the original documentation were released first in the two Lost treasures of Infocom collections, and all together in the Masterpieces of Infocom (bar leather goddesses)
