Worm holes - that tree's been dead (or mostly so) for a long time... Several types of insects and grubs will bore into wood like that - Carpenter bees, termites, powder post beetle to name 3 off the top of my head... Pretty common most anywhere that wood has been allowed to sit dead (or dying) with bark on and in ground contact. Some would say it's firewood (I often toss those in the fire pit pile) , others might say the worm holes could be turned into features after picking out the frass - holes can be left or filled with epoxy or whatever. (I might bother if the wood had a particularly unusual or interesting grain pattern, but find that such grain rarely shows all the way through.)
