Walt at the Explorers' League Outpost in the Howling Fjord wants you to use the Iron Rune Construct to explore the thundering cave at Baelgun's Excavation Site. Dismiss your construct by dismissing it from the pet window once you have completed the quest.
The cavern that we opened at Baelgun's was the one place that most steered clear of... Can't say I blame 'em. The sound of thunder shot out of that cave, [player]! Loud, booming thunder!
I think near the end, it was abandoned by all. Bad, bad things happened to any that went in to explore - most never coming back.
I want you to check that cave out while you're at Baelgun's. Explore around the titan statue inside and report back with your findings.