New commercial satellite imagery has provided the first definitive evidence that North Korea is dismantling its Punggye-ri nuclear test site ahead of a scheduled ceremony next week to mark the event, U.S. researchers said Monday.

The North Korea-watching 38 North website said in a report that imagery from May 7 showed that the process was “already well under way” at the site, which has been used for each of the North’s six underground nuclear test explosions.

It said several key operational support buildings, located just outside the north, west and south tunnel portals, had been razed, while some of the rails for mining carts, which had led from the tunnels to their respective spoil piles, had apparently been removed.