Wikia

An Underground House can be found underground within the dirt, dirt-stone and stone layer. They also rarely appear in oceans. It usually contains a Chest or Gold Chest, as well as 0-3 statues.

They are characterized by having a backwall of Planked Walls, which can be removed to show whichever background is directly behind it. The solid walls that surround the Underground Houses will always consist of wood and only wood. Underground houses have Wooden Beams in them. It is possible for underground houses to contain furniture, however, this is uncommon. They can usually be found around 1000 feet below on large maps, 700-900 on medium maps, and around 600 on small ones. Underground houses vary in size extensively, but are usually wider than they are tall; sometimes they are only big enough for just a chest. From time to time, they can have Pianos, Grandfather Clocks, Looms, Kegs and even Sawmills, along with other usually craftable-only furniture, although this is a rare occurrence.