D1: Descent Into the Depths of the Earth (1978)

First in the Drow series and follows directly after G3 in the Giant series. Written by Gary Gygax himself. It is a shorter module clocking in at just sixteen pages but the text is dense and there is not much filler in that text. The modular nature of the entire D series and its encounter and mapping allows the DM to add their touch to the series and make it as large and long as they desire.