Minecraft wiki entry for Nether_reactor, associated text: A nether reactor, ready to be activated.
A nether reactor after activation in 0.5.0, with the gold blocks still retrievable.
An active nether reactor spawning items and zombie pigmen.
A nether reactor upon completion of the cycle.
The Nether spire before 0.6.0, as seen from the outside.
A nether reactor getting excavated in Creative mode.