Lacunae

The Lacunae were gigantic tunnels that went to the hollow core of the plane of Mirrodin.

It was through each tunnel that the five suns, Bringer, Eye of Doom, Ingle, Sky Tyrant and Lyese, were ejected from the plane's core; extra mana of a particular color was sent off into the atmosphere.

Note
The Mirran Faction watermark on Scars of Mirrodin cards is a stylized representation of the 5 lacunae.