what were the earliest tomb structures on mainland greece called
tholos tomb (beehive tomb)
Beehive tomb
A beehive tomb, also known as a tholos tomb (plural tholoi) (Greek: θολωτός τάφος, θολωτοί τάφοι, "domed tombs"), is a burial structure characterized by its false dome created by corbelling, the superposition of successively smaller rings of mudbricks or, more often, stones. The resulting structure resembles a beehive, hence the traditional English name.

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Beehive tomb [SEP] Introduction
A beehive tomb, also known as a tholos tomb (plural tholoi) (Greek: θολωτός τάφος, θολωτοί τάφοι, "domed tombs"), is a burial structure characterized by its false dome created by corbelling, the superposition of successively smaller rings of mudbricks or, more often, stones. The resulting structure resembles a beehive, hence the traditional English name. Tholoi were used for burial in several cultures in the Mediterranean and West Asia, but in some cases they were used for different purposes such as homes (Cyprus), ritual (Bulgaria, Syria), and even fortification (Spain, Sardinia). Although Max Mallowan used the same name for the circular houses belonging to the Neolithic culture of Tell Halaf (Iraq, Syria and Turkey), there is no relationship between them.