While it definitely looks promising to use hydrogen for the no-carbon benefits, you do still have to wonder about other environmental impacts. What's going to happen to when the hydrogen pockets empty? Land subsidence could potentially be a problem, so I think more research about possible side effects is absolutely necessary before any large scale extractions happen.

Hydrogen may be a clean energy source, but extracting it would require fracking which is the polar opposite of clean. I have heard of some other cleaner way to extract it, but no idea if those methods are practical.

It is almost certainly greener than using more fossil fuel. This is an idea that should be explored.