nether portal

A manufactured structure that acts as a gateway between the Overworld and the Nether dimensions. A Nether portal is built as a vertical, rectangular frame of obsidian rock (4×5 minimum, 23×23 maximum size). The four corners of the frame are not required. Once a frame is constructed, it is activated by fire placed inside the frame. This creates purple, light-emitting portal blocks inside the frame, resembling a vortex. It produces whooshing noise regularly.
Such incomplete frames of obsidian are often found in naturally-generated ruined portal structures.

It can be destroyed by explosions, flowing lava or water into the portal block or by simply mining its obsidian frame.

There are also rumors about a variant of this portal using glowstone and water…