Descent Into the Depths of the Earth[2] is an adventure module for the Dungeons & Dragons (D&D) fantasy roleplaying game coded D1–2.

It was written by Gary Gygax, and combines two previously published modules from 1978, the original Descent into the Depths of the Earth and Shrine of the Kuo-Toa.

The latter segments of the campaign, including the D-series and module Q1, are set in a vast network of caverns and tunnels called the Underdark.

The plot of the original modules Descent Into the Depths of the Earth and Shrine of the Kuo-Toa places a party of player characters (PCs) on the trail of the drow priestess Eclavdra through the Underdark,[3] a vast subterranean network of interconnected caverns and tunnels,[4] battling various creatures on their journey.

[6] In D1 Descent Into the Depths of the Earth, the PCs seek the home of the drow by traveling through an underground world of caves and passages.

[6] The characters also find a grand cavern populated by drow soldiers, purple worms, a lich, a group of undead, a giant slug, sphinxes, trolls, bugbears, troglodytes, wyverns, and fungi.

The party encounters a kuo-toan rogue monitor who helps them cross a large river for a fee.

[8] The original modules Descent Into the Depths of the Earth and Shrine of the Kuo-Toa were both written by Gary Gygax and published by TSR, Inc. in 1978.

Other artists whose work appears inside the compilation include Jeff Dee, Erol Otus, David S. LaForce, Bill Willingham and Roslof.

[16] The latter segments of the campaign, including the D-series and module Q1, are set in a vast network of caverns and tunnels called the Underdark.

[17] The original TSR product codes for modules D1, D2, and D3 are 9019, 9020, and 9021 respectively (9021 being reused for the new Erol Otus cover with the blue background).

When combined with the G-series and Q module as the Queen of the Spiders, the D-series was voted the single greatest adventure of all time by Dungeon magazine in 2004.

"[3] Dungeon Master for Dummies lists Descent into the Depths of the Earth as one of the ten best classic adventures, noting that it takes "the player characters into the underground world of the Drow—the wildly popular dark elves of D&D lore.

He commented that the adventure would be too difficult for most groups of player characters, and speculated that buyers wouldn't actually play the modules because it would take a lot of sessions to finish.

He also thought that planning enemy tactics in advance was a good idea, providing an example of how to do so using the Drow outpost from D1, as well as other encounters.

Although he hadn't run D3 at the time of writing the article, he wrote that the thought of planning some of its encounters "brings me out in perspiration."

He was perplexed by all of the creatures found in the grand cavern area originally found in module D1: "All these creatures are apparently expected to do nothing but sit in their caves and wait to be attacked, for they would certainly defeat any moderately tough adventurer party specified by the author if they all tried a massed and well-coordinated attack.

"[6] Anders felt that the adventures contained a great deal of imaginative material, "but while the writing style is by no means poor, the combination of florid prose and new ideas makes a text which is relatively difficult to read.

Gygax does not make a regular practice of organizing his text to help the poor GM pick out the important information quickly.