In Stardew Valley, there are tons of hidden areas not really noted on the map explicitly. It’s up to you to fully explore and get to a bunch of these different locales. The sewers is one such area that you can totally unlock the entrance into.
In order to get into the sewers, you’ll need to get the Rusty Key after donating at least 60 items to the Museum. This will open up either the cover in town or the grates in the forest area to the south. Once you get within, you’ll be able to meet and talk to the vendor Krobus, who sells rare items. Also located within the sewers is the legendary Mutant Carp.
Now that you know how to get into the sewers, you’ll be swimming in rare stuff. Congratulations! And if you need any more help with Stardew Valley, check out our wiki!