Passage: The Yamnaya culture is identified with the late Proto-Indo-Europeans (PIE) in the Kurgan hypothesis of Marija Gimbutas. It is the strongest candidate for the Urheimat (original homeland) of the Proto-Indo-European language, along with the preceding Sredny Stog culture, now that archaeological evidence of the culture and its migrations has been closely tied to the evidence from linguistics and genetics. Significantly, there were animal grave offerings a feature associated with Proto-Indo-Europeans. The culture was predominantly nomadic, with some agriculture practiced near rivers and a few hillforts. Characteristic for the culture are the burials in pit graves under kurgans (tumuli). The dead bodies were placed in a supine position with bent knees and covered in ochre. Multiple graves have been found in these kurgans, often as later insertions. The earliest remains in Ukraine of a wheeled cart were found in the "Storozhova mohyla" kurgan associated with the Yamnaya culture.

Question: Multiple graves that were found were often referred to as what?
kurgans