For the destroyed remains of the Dark Portal briefly called the Rift during the Second War, see Dark Portal.
The Rift is all that remains of the first Well of Eternity. It is located in the center of the Eye which itself is located in the center of the Maelstrom. At the end of the War of the Ancients, the powers released in the Well of Eternity blasted the well downward into Azeroth's molten core. 10,000 years later that core is still exposed. The well's descent broke an enormous fissure in the ocean floor. The red and orange light from lava is visible deep, deep down. It is held sacred by the naga who are capable of reaching its depths as the pressure does not seem to affect them. The seafloor around the Rift is noted for its thermal activity. Nazjatar is constructed on the sides of the Rift. Although Brann Bronzebeard could not go too far down into the Rift because of the pressure, he said that "Perhaps the Well of Eternity still exists, in some form, in Azeroth’s heart, for arcane currents emanate from the Rift."
The Rift is also the prison of the Old GodN'Zoth, the being responsible for Deathwing's madness and the creator of the Emerald Nightmare.