Burrow - The target creature goes "Underground". Underground creatures can only be attacked normally by other Underground figures. If an underground creature attacks a non-burrowed creature, it can only deal 2 damage max. An "underground" creature can "unburrow" at the cost of dimming.

Mole
1 MF
3 Flux
1 2/2 3 3 0
Burrow
When burrowed, Mole's vision becomes 0.

Drilling Explosive
2 MF
1 Flux
5 1/1 5 0 0
Lifespan(3)
Burrow

Druid of the Earth
1 SYL
4 Flux
1 3/3 2 4 0
(3 Flux) Once per turn, provide 1 MF domain for the rest of the round.
(3 Flux)Burrow

Rock Golem
1 ELE
3 Flux
1 4/4 3 1 0
Burrow

Mole's Claws
1 MF or 2 UA
2 Flux
Enchant Target Creature
Draw a card when you play this enchant. Enchanted creature gains Burrow. If Mole's Claws are removed while the creature is still underground, it unburrows automatically.

How about: the ability to "Burrow" allows the creature to become "Underground" which acts like an inverse of Flying/Aloft, except also immune to range attacks.

Cool suggestion. I've changed it, but I've limited the attack possiblities as well otherwise all burrowing creatures would be near invincible to other creatures.

The whole 'underground' idea is well in line with the rest of the game mechanics of TFW, so I think this could be an interesting development.

As we know, factions are 'theme-based' rather than 'lore-based' (for the sole reason that noone decided to develop the lore beyond the bare minimum?), yet 'Burrow' as a trait, or perhaps 'Underground' as an ability, fits better with some factions (Elementals) rather than others (Sylvan). So it wouldn't be surprising to see some factions benefiting more from this than others, with the usual counter-cards in place.

For example, if Sylvan is limited in the kind of Underground 'stuff' they can use, perhaps they can have traits such as 'Observer' where, say, a Thistle Falcon has the ability to detect underground creatures. The ability to Observer can function just like Vision, yet the Falcon might have better/worse Observation than Vision.

And along these lines, you can steal much more from SC2