Nether Portal

A  Nether Portal is a player-built structure which, when ignited by Flint and Steel, creates a portal to the Nether dimension. It will be added in Update 0.12.0.

Construction
A Nether Portal can be constructed primarily by placing Obsidian in a 3x4 block rectangle, however, it can also be created in upto a 23x23 size.

Trivia

 * The Nether Reactor served as a temporary replacement to the Nether uptill Update 0.11.0.
 * It is unknown whether mobs will be able to pass through it or not.