Radix

The Radix was one of the five lacunae on Mirrodin. Originally, it appeared as a wide circle of brilliant metal, somewhere near the middle of the Tangle.

[SPOILER ALERT]

However, once Glissa tapped a large amount of mana to destroy the Kaldra Champion, a surge of mana in the Mana Core itself discharged through the Radix to become Mirrodin's fifth moon, effectively forming the Green Lacuna.