[Susiron]

As Big said, those are just mining tunnels. They're very common around levels 13 to bedrock, and it's very hard to find any stretch of land that doesn't have some form of mining tunnel running directly under it, or nearby. Unless you run into a player-made (or player-claimed) structure underground, tunnels are completely fine to dig, even under claimed land (though I think it would be frowned upon to intentionally dig a few blocks beneath someone's structure-- but if you're just a spit above bedrock and not intentionally creating a secret burrow under someone's house, it's non-consequential ahahaha