Does a creature provoke opportunity attacks when it leaves a threatened square adjacent to an enemy by burrowing?
See this closed question for prior discussion on the topic of burrowing.
Opportunity Attack ddi
Moving Provokes: If an enemy leaves a square adjacent to you, you can make an opportunity attack against that enemy. However, you can’t make one if the enemy shifts or teleports or is forced to move away by a pull, a push, or a slide.
Exception does not include burrowing. But more importantly:
Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action.
The interrupt (not a reaction) is key. So before the burrowing happens, the
Opportunity Attack happens.
There is some clarity in the power description:
Burrowing Speed ddi
A creature that has a burrow speed can move through loose earth at a specified speed, and the creature can move through solid stone at half that speed. The creature can’t shift or charge while burrowing.
The reason to prohibit shifting during burrowing is to explicitly allow for
... most likely ... No
This is assuming the creature is in an adjacent ground square, not in an adjacent open air square, in which case the answer is ... Yes
Unless the square of ground the monster is burrowed in is transparent, there is no
Opportunity Attack [ddi]
Able to Attack: You can’t make an opportunity attack unless you are able to make a melee basic attack and you can see your enemy.