Archaeologists have unearthed some ancient monumental structures in different places that manipulate sound for sensory and mind effects. These ancient structures with particular designs have been found in Ireland, Malta, southern Turkey and Peru. Researchers believe they might have been specially designed to conduct and alter sound for producing certain sensory effects. The underground temple Hal Saflieni Hypogeum on the island of Malta, for example, is a 6,000-year-old stone structure complex with manipulated sound design. The monument includes central corridors and curved chambers with a complicated design, in which low voices within its walls create eerie, reverberating echoes, and a sound made or words spoken in certain places can be clearly heard throughout all of its three levels. Scientists believe that certain sound vibration frequencies that are created when sound is emitted within the building’s walls alter human brain functions of people within earshot. A study conducted in 1994 by a Princeton University consortium confirmed similar peculiar sound effects in ancient chambers of megalithic sites such as Newgrange in Ireland and Wayland's Smithy in England. Experts suggest that peculiarity of sound effects varies with dimensions of the room and the quality of the stone.