Built between the third and seventh centuries, kofun are ancient Japanese burial mounds that served as elaborate tombs for powerful clans. The most common form of kofun was the zenpō-kōen-fun, a massive tomb erected in the shape of a keyhole. While it’s estimated that there are over 160,000 kofun scattered throughout Japan (with 700 in Tokyo alone), they’re quite difficult to spot; to the untrained eye, they may look like mere mounds of earth.

For this reason, the burial mounds of the Hodota (or Hotoda) Kofun Complex in southern Gunma are anomalies. With the help of a volcano and some significant restoration work, they look almost exactly as they would have looked 1,500 years ago. When nearby Mount Haruna erupted in the sixth century, its thick ashes blanketed the three mounds that make up the complex, preserving them for over a millennium until they were unearthed in the 1800s.

The complex’s three kofun are massive, carefully preserved, and well-maintained. The Futagoyama Kofun, the largest of the three, is about 108 meters (118 yards) in length and 10 meters (32.8 feet) in height. Yakushizuka Kofun, the second mound, is now part of Saikō-ji Temple, and only a small portion of it is visible today. The Hachiman-zuka Kofun was home to 400,000 fukiishi, or roofing stones, and 6,000 cylindrical haniwa, or terra-cotta clay figurines. These haniwa typically depict humans, animals and houses, offering tangible snapshots of everyday life during the Kofun period. Today, replicas of such haniwa surround the complex en masse, some bearing shields as if to protect the restored burial site.