Is it true? my question is for thunder to fly, surf to dive, and earthquake to dig... does it double the damage?
Yes, Earthquake (and Magnitude as well) will deal double damage is the opponent is underground on that turn.
Thunder has 100% accuracy if the opponent is using Fly, but it does not do double damage. However, it does double if the opponent used Bounce.
Gust and Twister will do double damage to pokemon that are using Fly or Bounce.
If you use Dive then Surf and Whirlpool will do double damage.
Yeah, it inflicts double the damage. Pretty neat, huh?
The same for Fly will also go with Gust, Twister, and Submission.
Dig also reacts that way to Magnitude.
And I think Dive reacts to Whirlpool, as well, but I'm too sure about that one. =P