How to Get the Rusty Key & Unlock the Sewers in Stardew Valley

Talk to Gunther in the Museum.

Accept his task to donate any valuable items you find. Gunther is an NPC (non-player character) who runs Pelican Town's museum in the game. Walk over to the museum in the southeastern corner of Pelican Town—you can do this as early as you like in the game. The museum will be empty at first; donate artifacts and valuable items to fill up the museum and be rewarded with the Rusty Key. To get the Rusty Key, you'll need to find and donate 60 items to the museum. It may seem like a lot, but there are 95 items to find in total—so it can be done! If you're not sure whether an item can be donated, check its description in your inventory. It'll say, "Gunther can tell you more about this if you donate it to the museum."

Look for artifact spots in every outdoor area.

They look like 3 wavy lines on the ground. One of the easiest ways to collect artifacts for donation is to simply look for signs of an artifact spot, known as worms, stems, or twigs since they wiggle in the game. Artifacts have the highest chance of spawning in winter, although you'll be able to find them year-round in the game. The type of artifact you find depends on the map you dig it up in. Potential artifacts you'll find this way include fossils, anchors, and ancient or prehistoric items like dolls, instruments, jewelry, and weapons. You'll need to dig artifacts up with a hoe, as other tools won't work.

Dig in the mines.

Use your hoe to find fossils and minerals in these areas. You can also donate minerals and gems (found inside Geodes) to the museum, and you'll have the best odds of finding these while mining. Go to the mines located in the north area of the mountains and explore them to find minerals and geodes containing gems. Keep in mind that the mines are inaccessible until the 5th day of Spring (Year 1). You'll get a letter informing you when you can visit the mines. The mines are large, containing 120 levels total with increasingly valuable ore on each descending level. To get to the next level, find a ladder somewhere on your current level of the mines.

Visit the Skull Cavern.

You can also mine for minerals, gems, and special items here. To get to the cavern, you'll need to find the Skull Key on level 120 of the mines—so clean those areas out before you come here! Then, proceed to the northwest area of the Desert, and you'll find the Skull Cavern. You'll progress much like the mines, as the cavern also requires finding a ladder to progress to a new level. In addition to ore and gems, you'll also be able to locate artifacts by fighting dinosaurs on the prehistoric floors of the cavern.

Kill monsters.

Some artifacts are only available as loot when you kill a creature. For the most part, you can find monsters in the mines, the Mutant Bug Lair, the Quarry Mine, the Skull Cavern, and the Volcano Dungeon in Stardew Valley. You can also find monsters at night on the farm, so long as you toggle them "on" at the beginning of the game. When killed, monsters drop loot, some of which can be donated to the museum. For example, defeating monsters is the only way to get the game's Dwarf Scroll collection, unlike other artifacts. To slay monsters, make sure your character has a weapon (especially an enchanted weapon), food items that give buffs, and good mobility. Time your weapon strikes so you can push away or damage monsters effectively.

Fish for treasure chests.

They sometimes contain artifacts and other resources. Fishing in the game normally allows players to catch different aquatic creatures; however, you can also find treasure chests while fishing throughout the game. Fish regularly to catch as many treasure chests as you can. Some chests need to be caught in higher fishing zones, so ensure you get your bobber in each zone while fishing. In Stardew Valley, fishing spots are ranked from zone 0 to zone 5 (5 being the best zone). The further away from land in every direction, the better the zone. A zone 5 space is at least 5 tiles away from land.

Watch the Fortune Teller on TV.

Have your player character watch TV in their farmhouse every morning. The Fortune Teller will update you on your character's Luck for the day, which is significant as the Luck stat affects the drop rate of treasure, geodes, and gem nodes. When the Fortune Teller says that the spirits are "very happy" or "in good humor" that day, you'll have a little extra Luck. Go out looking for treasure on lucky days. Conversely, when the spirits are "somewhat annoyed," "mildly perturbed," or "very displeased," you're less likely to find special treasures in the game.

Break open bone nodes.

Search for these at the Ginger Island Dig Site. Once you unlock Ginger Island by repairing the boat at Willy's Fish Shop and buying tickets, visit the island dig site. There, you'll be able to find rocks, bone nodes, clay nodes, and mahogany tree seeds for collecting. Upon breaking open bone nodes with a pickaxe, you'll have the chance to find fossils and bones in demand at the museum. You may also collect bone fragments from a node, which is useful because they can be used in crafting, and Gunther will make a special order for you to collect 100 fragments at some point.

Open artifact troves at the Blacksmith.

Collect containers with valuable items called artifact troves as you play. You can buy them from the Desert Trader, collect them from Haunted Skulls, dig them up from artifact spots, or find them in supply crates. Bring any troves you find to Clint the Blacksmith in Pelican Town and pay 25 gold to have him break them open so you can claim their contents.

Donate 60 items to the museum.

Enter the museum and speak to Gunther to make a donation. Once you donate 60 items to the museum, Gunther will show up at your farm the next morning and give you the Rusty Key as a reward. He'll describe it as an old key that has been sealed up in the museum vault for a century. After receiving the key, you'll be able to access the sewers in Stardew Valley. There's no need to donate all the items simultaneously, especially since you have limited inventory space. Donate to the museum during its open hours (8 AM to 6 PM) as often as you want.

Unlock the sewers with the Rusty Key.

Use entrances in Pelican Town or the Cindersap forest. The Pelican Town entrance is near the center of the map, in a fenced-off area to the left of the graveyard; approach a square cover to unlock the sewers here. In the Cindersap forest, go south past Marnie's ranch, cross a wooden bridge, and keep south until you see a sewer grate. From there, you can unlock and head inside!