Ancient tombs are easily found from the sky. They appear as a large mound covered with light blue flowers. To “collect” each one you must Pay Respects by finding the door and holding Y/Triangle. An Urga Stupka-210 (handheld mortar) is found behind each tomb after you do this. Once you’ve Paid Respects at all of the Ancient Tombs, the Urga-Supka-210 becomes available for Rebel Drops.
Just Cause 3 eGuide Collectibles Ancient Tombs