Not on earth could mean dry land. So maybe they'll be in the sea practicing their swimming with millstones.
(Matthew 18:6) . . .But whoever stumbles one of these little ones who put faith in me, it is more beneficial for him to have hung around his neck a millstone such as is turned by an ass and to be sunk in the wide, open sea.
Or maybe they'll be wishing they could go swimming with millstones, as this is the preferable option for those who stumble the little ones who have put faith in Jesus Christ.