The owner states the barn it was in had a leaky roof...the barn must of been at the bottom of a lake.
https://www.ebay.com/itm/1966-Ford-Bronco-chassis-with-TITLE-solid-frame-rusted-from-leaky-barn-roof/333551820374?hash=item4da93cde56%3Ag%3AznYAAOSwzCFec97b&LH_ItemCondition=4
https://www.ebay.com/itm/1966-Ford-Bronco-chassis-with-TITLE-solid-frame-rusted-from-leaky-barn-roof/333551820374?hash=item4da93cde56%3Ag%3AznYAAOSwzCFec97b&LH_ItemCondition=4