Sewer

Sewers are underground Structures similar to Strongholes found in the Overworld. They are filled with Stone Bricks (including Mossy and Cracked varient).

About
Sewers have Cobwebs, Vines, Boats and Streams. There are also Torches there. There are also Silverfish Blocks there and sometimes monsters (especially slimes). You can also find Trapped Chests their. There are Diamonds, Cantaloupe Seeds, Melon Seeds, Gold Ingots and Redstones you can find in the chest. You can also find a Calfloward portal there as well.

Mobs

 * Slime
 * Bat
 * Silverfish
 * Skeleton
 * Zombie (sometimes)
 * Creeper (sometimes)
 * Cave Spider
 * Spider (sometimes)

Trivia

 * Similar to a Stronghole.
 * The water transportation goes through Block of Irons out of the Sewer.