Sewers

The Sewers in Stardew Valley can be entered from south of the square in Pelican Town and south in Cindersap forest (south of Marnie's Ranch).

Krobus's Shop

Inside the sewers, you’ll find the merchant Krobus selling rare items for extremely high prices. Some of those items are Monster Fireplace, Void Essence, Solar Essence, Void Eggs, and a Stardrop.

Mutant Bug Lair

After completing the Dark Talisman Quest, you’ll gain access to the Mutant Bug Lair. The Dark Talisman Quest will be given to you by The Wizard.

Statue Of Uncertainty

The Statue Of Uncertainty allows you to change your profession perks at the cost of 10,000g.

How to Get Sewer Key

To get the Sewer Key, or “Rusty Key” in Stardew Valley, you must donate 60 or more items to the museum. It doesn’t matter which particular items you donate as long as you hit the 60 mark.

Once you reach 60 donated items, the next morning you will see a cutscene play where Gunther thanks you for all your donations. Right before he walks away, he turns back around and gives you the Rusty Key. The Rusty Key or Sewer Key can be used to unlock the sewers.