That is simply wrong. Underground storage is not a panacea. Few mines are suitable and even those require expensive preparation and maintenance for coming decades or even centuries, with many hard to predict risks (think earthquake, flood).
Few mines is all we need. The entirety of the USA would need just one such site.
There's plenty of nasty natural stuff underground and a well-designed mine storage site would be a lesser risk than continuously radioactive coal power plants of today.