None, I think. If you can build just one per solar system, I think it should be saved for interplanetary transport. Maybe we could build a base in Europa or something.
Clarification for how it works. A building has two bay doors on it. When you go through one you walk out the bay doors in another building. You can see across it like you would in portal.
The room that hosts these portals takes up about 5' into the building. But you can only build one per solar system, or gravity and space time get swapped for each other and that's not good.
Maybe put one end in the middle of gold country in the Amazon and the other somewhere in America. A big problem of getting the gold out of the Amazon is getting the equipment in.
My office to the gold reserve vault.
My basement, and another Earth like planet, access to another earth worth of resources is the most economic value you could have I would think.
Put one under a drop door on the floor of the Oval Office and the other over a volcano
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