The base.

The base wouldn't be so much of a support as it will be an anchor.

The air inside the dome could differ from the outside so much, that it would just float away! So to keep that from happening, we would need a concrete ring for the base, weighing in at 300,000,000 tons of concrete overall, and circling about 3.1 miles. This would take ten years of the project, but would be essential to the domes success.