Trial Gate

Trial Gates are difficult to mine, especially while you have Mining Fatigue in an Ocean Monuments. To get rid of a Trial Gate blocking your path, get close to it so its mouth opens, and right click it while holding a Gate Key.

When opened, any Iron Bars connected to a Trial Gate will be destroyed as well. There is a limit to this to avoid large amounts of lag, but this limit will almost never be encountered, as Trial Gates will never be found with that many Iron Bars joined to them.