The Undergate was the doorway that blocked the left-hand side passage from the main tunnel in Shelob's Lair. It led to the Under-way, a winding tunnel that ascended to the base of the Tower of Cirith Ungol. The gate itself was a great block shaped like a heavy door less than twice Sam Gamgee's height. Above it there was a dark blank space between the top and the low arch of the opening.
When Frodo and Sam first encountered the Underway they were in darkness and only perceived that the way was blocked. Later the orcs from the tower carried Frodo through the opening but Sam, following behind, found the doorway shut. After hearing Shagrat declare that Frodo was still alive and after the orc-captains had gone, Sam discovered by the light of Sting that he could get over the blockage by leaping and scrambling over. Unable to enter the base of the tower, Sam returned to the Undergate and still not finding the means to open it climbed back over to scurry through the main tunnel to the tower to rescue Frodo.