they clearly held some sort of gas fine for millions of years, so certainly they’re gonna store a bit of hydrogen too.

Not to rain on your parade, but hydrogen and natural gas aren't really comparable for storage. The natgas molecule is 8x heavier and MUCH larger than a molecule of hydrogen. Just on the size alone, hydrogen can slip through just about everything and needs to be stored at cryogenic temperatures. I don't think rock is going to be as good of a storage media as you'd assume.

Oh that makes sense.

We just don't have any natural gas production in Estonia lol. Perhaps the shale mines could be used. Unfortunately the biggest one had its permit extended till 2049 recently. Also I think they get filled with water naturally (they pump out a lot of dirty water), so I suppose the walls aren't actually completely sealed naturally.