The practice of constructing burial mounds, called "kofun," flourished from the 3rd to 7th centuries AD throughout the Japanese archipelago. There are over 160,000 kofuns, including the keyhole-shaped mounded tombs with rounded rear portions that complete a keyhole shape as seen from above, as well as mounded tombs with round or square shapes. In that era, kofuns served as monuments that displayed one's status and power, which was reflected in the tomb's size and shape. All such tombs were constructed solely by manual labor during the Kofun period, which stretched across centuries and marked the first widespread completion of civil engineering works ever in Japanese history.