Now that I have a real keyboard I'll tell you everything.

First, Idaho (and Maine, and the borders of Ireland) were once high, flat plateaus.

Secondly, the "potato" does not grow as a tuber on a plant. It is more like a nematode crossed with a grub.

The "potato" has as its favorite food the plant Solanum Tuberosum. It attaches itself to the roots of the plant and, in a symbiotic relationship, thrives. It can sense the presence of the plant through the soil and will migrate at least several miles (or kilometers or kilometres) to reach it.

The relationship between plant and animal continues until the plant dies from the end of the growing season or from so other cause. If the demise of the plant is due to the end of the growing season, the "potato" turns into the starchy "tuber" we know.

Some soils (such as those in the areas mentioned above) are particularly conducive to the growth of Solanum T. and hence to the "potato." As the plants were domesticated for farming, the "potato" would, naturally, come to them. This left hollows in the soil when enough "potatoes" had migrated. These hollows would, of course, be filled in with soil.

Over the course of time, enough "potatoes" migrated due to farming that the collapse of the soil reduced the high plateaus and gave birth to the mountains of Idaho, Maine, and the coastal areas of Ireland.

And now you know The Truth. The "potato" is actually a pretty stupid animal, a root sucker. Keep this in mind if you are a vegetarian or worse.