Complex sentence: In the Japanese archipelago, tumulis (kofuns), which are mounds of earth and stones erected over graves of the ruling class, numbering more than 20,000 were built as monuments during the later part of the 3rd century and extending to the 6th century.
Split into simpler sentences: In the Japanese archipelago, there are tumulis (kofuns), which are mounds of earth and stones erected over graves of the ruling class. More than 20,000 were built as monuments between the later part of the 3rd century and the 6th century.