Ley Lines were thought up, or discovered, by Alfred Watkins author of the 1925 book 'The Old Straight Track' and the later 'Early British Trackways'. He believed that they were an ancient network of tracks for prehistoric travellers. He explained that they ran between sacred sites and places of power--standing stones, burial sites, and settlements were constructed along the lines, and particularly at junctions of multiple leys.

There was a brief wave of enthusiasm for this theory, and clubs were founded all over the country to explore ley lines and their significance. The fad soon faded.

The ideas were revived in the sixties and since, and are common amongst those who cherish crystals and take crop circles as signs from the Alien Gods. In more recent years, people have associated ley lines not with travel, but with psychic power, spirits, fairies, or dreamtracks. (This strand of belief started in 1936, with the occultist Dion Fortune. Ties with UFOs only began when UFOs became fashionable.) There have been attempts to link these ideas in with ceremonial sites and practices around the world (e.g. the australian songlines). Modern believers get very excited by the imagined links to shamanistic practices, and the ideas of spirit paths, and try to explain that Watkins was also discussing Native American spirit roads, not just flattened meadows in England.

Even magazines like 'The Ley Hunter' struggle to make the links they desire:
Nobody knows for sure, but it seems that earlier peoples considered that straight landscape lines to facilitate the passage of spirits. Research by Paul Devereux and Nigel Pennick has revealed that a history of the straight landscape line can be seen in which it moved from being a sacred or magical thing into the secular sphere. Leys, old straight tracks and "terrain oblivious lines" or whatever you want to call them, reveal a deep mystery lodged as much in the human psyche as in the landscape. Current research is revealing more and more examples of linear trackways, roads, paths and mythological routes which are connected to the dead, spirits of the dead and spirit travel.
Deveraux's ten years of research, known as The Dragon Project, demonstrated there were no special energies or effects along ley lines. The research was rigorous: it employed dozens of amateur dowsers, psychics, sensitives and spirit photographers. Even those looking for proof could not find it.

It's easy enough to find a ley line: take an Ordinance Survey Map, a pencil and a ruler. Find a few significant ancient sites, and start drawing lines between them. Britain is littered with traces of ancient settlements: draw a line long enough and it will cross through dozens of them.

Once you have found a set of nice straight lines that look attractive, go to the area and start walking around. Look for features in the landscape: notches in the hills that line up with your point of view as you hike along the path, interesting rock formations, or woods that keep your path straight. Check to see if there is any flattening in the path you are walking. (If not, stamp extra hard to leave a trail to convince others who follow your path.)

If ley lines are remnants of Stone Age tourism, you would assume that they are the shortest route between two points. Looking at maps of 'established' ley lines, however, this only holds if prehistoric man had the power of flight: some of the routes are impossible to walk.

Ley lines are fiction.