Underneath means directly below something or covered by it. When you hide a note underneath your pillow, it's tucked between the pillow and the sheets. When a cat crawls underneath a porch, it's in the space below the wooden boards.

The word suggests being hidden from view or protected by whatever's on top. If you wear a T-shirt underneath your sweater, people can't see it. Archaeologists dig underneath ancient cities to find artifacts buried by centuries of dirt and debris.

Underneath works as both a preposition (underneath the bridge) and an adverb (the bridge with water flowing underneath). It's more specific than just “under” because it emphasizes the close relationship between the two things.

You might also encounter underneath used metaphorically: “Underneath her calm exterior, she was nervous about the performance.” Here it means beneath the surface that others can see, describing what's truly there when you look deeper. It's the difference between what appears on top and what's really there when you look closer or dig down.