"Dolmen" is a word that is used to describe certain megalithic (made out of giant stones) structures that exist world-wide, which no one has any really good explanation for. A dolmen can, in it's simplest form be described as three large rocks, formed like the Greek letter Pi. That is, there are two vertical stones on the side, forming two walls, with another single stone laid across the top, forming a roof. These structures are also called such things as portal barrows, portal tombs, tunnel...