Wormhole

While it's certainly possible that this is is literally a "hole made by a worm," it's also a wormhole back in time. During the carboniferous period 350 million years ago, mosses dominated the landscape. Trees hadn't evolved yet, so the forests were populated by huge clubmosses the size of trees. Insects were also bigger than man in some cases, so if you were somehow transported to that time and came upon a large cave as depicted in this scene, you may not want to enter for your own good.