h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There's speculation that the spirals on the stones indicate that the use of Stonehenge was for an inward journey and that there was a special quality of sound inside the inner circle.

Some interesting stuff here about the possible use of Stonehenge to gain altered states here:

'There are many speculations as to the meanings of these spirals, but the one that is most relevant is the symbolic meaning signifying the entrance into another realm of consciousness. Spirals have long been associated with energy patterns found by geomancers around the earth. Some have dowsed around the stones at Avebury and Stonehenge and have found spiraling energy at each stone marker. Other sacred places such as at Sedona, Arizona, have been sought after because of the vortexes or spiraling energy that is said to take the pilgrim or visitor into altered states of consciousness. In the 1977, Paul Devereux and John Steel, authors of Earthmind, founded the Dragon Project to better understand the earth and her emanations. Using Geiger counters, ultra sound and other instruments to measure the magnetic frequencies at different places on the earth, specifically sacred sites, they found that the instruments registered higher in frequencies at the sacred sites than at other locations at nearby sites. They have speculated that the granite around the areas is able to "hold" the frequencies, which have been enhanced through ritual and prayer. They also found that granite has the capacity to attract and emit radioactive fields. Perhaps this is why Stonehenge and Avebury circle is thought to hold frequencies that can be felt in the stones by those who have developed inner sensory perceptions.'

from the soon to be published book 'Threshold Places by Constance S. Rodriguez PhD, LCSW

Full excerpt here: http://www.themetaarts.com/archives/200304/connier.html

This also has some fascinating speculation:

http://www.rsm.ac.uk/new/pdfs/stonehenge.pdf