The location of "they" depends on the context. Without more information, here are some possibilities:

• Volcanoes Most volcanoes are located around the Pacific Ring of Fire

. They can also be found along the west coast of North and South America, at destructive and constructive plate boundaries, and at hot spots on the earth’s surface

• General locations They can be generally located where there is a known crash risk

. They are scattered and not strategically located to minimize transportation from dock to plant