Chance should be equal with all other places. Whenever you explore you have 33% to Forage/Discover Nothing, 33.5% to Encounter a Monster and 33.5% to discover a location or event. Whichever event/place you find is equally random across the board. There's 4 events and 4 places to discover in the plains.
The place you want to discover is called Westdale Caverns. I'll add another way to get Iron in the next patch, maybe buying it from the blacksmith or something.