30 is a square pyramidal number i.e., the sum of the first four square numbers (1² + 2² + 3² + 4² = 30), as illustrated below:




ARCHAEOSOPHY OF STONEHENGE III - THE EARTHSPHERE

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The best known feature of Stonehenge is the Sarsen Circle which was built as 30 upright stones with 30 lintel stones on top of them.