Satellite images have captured what appears to be tunnel entrances leading to Iran’s new secret underground nuclear site, The New York Times reports. The facility is believed to be a direct replacement for the centrifuge assembly hall at Iran’s main nuclear fuel enrichment complex that was destroyed by a mysterious explosion in July. At the time, Iran blamed the explosion on sabotage and the head of Iran’s Atomic Energy Organization, Ali Akbar Salehi, pledged that the new building would be set in “the heart of the mountains” to protect it. The satellite images show the progress of that mission. Jeffrey Lewis, an arms-control expert at the Middlebury Institute of International Studies, told the Times the images appear to show two tunnels linking a vast new space that is big enough to replace the destroyed centrifuge assembly hall.