ley lines are made up of various kinds of energy; whatever kinds are more prominent in the line affects its properties. There are, for example, ley lines with defensive, disruptive, or restorative energy.
They can be tapped into by magic users to draw power from them. Using a ley line is the most powerful means of employing magic known to mortal wizards.
Some might be more stable which means that it is better to use when a stable flow of energy is needed. Others might have destructive energies, which means it is better to use in battle. The island Demonreach has a ley line that has certain malevolent properties. This makes people who go near the island unconsciously veer away.
Ley lines with heavy concentrations of dark energy run through both North and South Manitou Island in the north of Lake Michigan, and the secret islands in the lake's center, including Demonreach. Another ley line runs directly beneath the Field Museum in Chicago.
The ley lines that intersect deep in the ground under the Auld Rock and the White Council headquarters contain a "staggering amount of raw power" and it's one of the world's largest ley line convergences. These ley lines are used in order to create super powered ward without a threshold covering the entire building. Presumably the wards constantly draw from the ley lines if strained.
Another large confluences of ley lines is located at Chichén Itzá, in Mexico. It's one of the biggest in the world. The Red Court stashed their ritual gear and artifacts near ley line confluences. 
In the seriesEdit
In Small Favor: the Order of the Blackened Denarius used ley lines to empower a circle prison to contain the Archive on the island of Demonreach. They would not have been able to hold her without the strength of the ley line assisting with their super circle.
In Changes, soon after arriving in Chichén Itzá, Harry Dresden felt the ley line that ran under his feet and found that it held an enormous current of raw earth magic. He tapped into it and was overwhelmed by the rush of imagery and sensations, of the earth plates creating mountains for one. He pulled out some of it's power and released it in a spell—gravity vanished. The horde of vampires that had been charging them left the earth and were smashed back down again, killing most of them.
In Cold Days, Harry Dresden started to mark the location of nodes—points where ley lines intersected on a map of Lake Michigan; Molly Carpenter stated that Auntie Lea taught them to her few months prior.
- ley line - Wikipedia
- ley LInes — Skeptic Dictionary
- ley LInes — Society of ley Hunters
- ley-lines — Ancient Wisdom
- Historical Geodesy — Geolines
- leylines of Earth Energy: Earth Magic
- Lake Michigan - Wikipedia
- Edinburgh - Wikipedia
- Chichen Itza Facts , Mexico
- Across the Coils of the Feathered Serpent | Intrepid Blog
- Vortexes: www.OfSpiritandSoul.com
- Liminal Thresholds: ley line maps
- Vortex Maps Site Guide