Why would anyone in their right mind pay a million dollars for a claim under someone's house?

Historically the claims were there before the houses. Van Dyke sold the land to the miners to entice them to come and live near the mine and build a community. When he did it, he retained the right to drill 40 feet deep and below their properties. It has always been on title to their homes because it existed before the town and was the only reason the town was even built.

The more recent court case solidified the arrangement but changed it so that the mining couldn't start above 500 feet.

The question really is, why would someone buy a home over a legitimate mine? I'm guessing it was very cheap, but clearly the arrangement stood up in court to a great extent. Like it or not those houses always had a mine underneath them.