Ōmuro Kofun group
Overview. The Ōmuro Kofun Group is a large necropolis consisting of over 500 tumuli spread across two valleys on the south side of the Chikuma River at an elevation of 350 to 500 meters approximately six kilometers southeast of Nagano city. The tombs were built over a 250-year period from the 5th to the 8th centuries. They have been grouped by archaeologists into five groups ( Kanaiyama, Kitatani, Kajo, Omurodani, and Kitayama) by their geographic location, stretching across an area of roughly 2.5 square kilometers. There is only one keyhole-shaped "kofun" (), but at least 330 smaller  circular-shaped "kofun" () made of stacked river stones from the Chikuma River. Most of these circular-shaped "kofun" have a diameter of approximately ten meters, and there is no other burial mound cluster in Japan where there are so many such circular-shaped "kofun" in such a small area. These mounds can be divided into 40 different variations, some with a scallop-shaped stone burial chamber, or with a triangular gabled burial chamber ceiling. Theories that these tombs were built by immigrants to Japan from the ancient Korean states of Goguryeo or Baekje remain controversial. Excavated grave goods include Sue  ware and Haji ware pottery, bronze mirrors, armor, swords, horse fittings and jewelry. The number of horse bones found was unusually large, and included the skull of a horse which was buried in the vestibule of one of the horizontal burial chambers. The site is now an archaeological park with a museum, the displaying some of the artifacts discovered.