The Underground Passage is one of the first unknown locations you'll find in Diablo 2.
As part of the Search for Cain quest in Act I, this area requires some exploration beforehand. Considering that this quest has many steps to go through, saving as much time as possible is key.
This page explains the Underground Passage location in Diablo 2 so you can complete Search for Cain promptly.
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How to start Search for Cain in Diablo 2
In Diablo 2, Act I serves mostly as an introduction to both the story and some of the key characters that you'll meet along the way. This includes the well-known Deckard Cain, who happens to be trapped in Tristram during the first few hours in the game.
Search for Cain is the third quest in the game, and is granted by Akara in the Rogue Encampment after completion of Sisters' Burial Grounds. It's quite a lengthy one, requiring you to get through multiple areas to find a scroll, activate a portal to Tristram, and rescue Deckard Cain.
This quest starts in the Cold Plains, which is the first area of Act I. You will then have to find your way to The Stony Field, which is one step closer to the Monastery Barracks and Monastery Catacombs.
Sadly, map layouts in Diablo 2 are randomly generated, which is why we can't show you the exact location. But don't fret - the following sections will explain the usual path regardless of the variant, as well as a few screenshots that can provide a helpful reference.
Looking for more help? We have pages on the best Diablo 2 class, best Diablo 2 builds and runewords, Diablo 2 leveling, how to import saves, how to reset your skills, how to identify items, when and how to imbue items, where to find monastery locations and the underground passage, and finally, how to access the secret cow level.
Diablo 2 Underground Passage location explained
Heading from Cold Plains to The Stony Field is simple enough. The road will branch towards it, so you won't have a hard time finding it.
As per usual, make sure to search for the waypoint once you're on site, so it's easier to backtrack and move across areas later on.
Finding the Underground Passage may take some exploration. This is a tough area early on, so expect to encounter multiple enemy groups awaiting your arrival.
Luckily, there's a key point of reference that can be helpful. The Underground Passage in Diablo 2 is always found on the limits of the map, following the walls.
This narrows down the search exponentially. There are areas later on that require for you to search the entry across different spots on the map, but that's no the case here.
While it's not possible to guess where exactly the entrance will appear, your safest bet is to stick to the walls until you eventually stumble upon it.
Once you're ready, go ahead and enter. The Underground Passage has two levels - you can get to the Dark Wood from the first to continue the Search for Cain mission, or keep on exploring the floor beneath for some loot. In either case, you're able to come back here later on.
That's all you need to know to find the Underground Passage. Best of luck bringing Deckard Cain back safely!