Existing roads, highways, streams, rivers, and wetlands are all traversed by pipelines. Pipelines are often built beneath these obstructions by boring to a shallow depth or by horizontal directional drilling (HDD) for deeper installation. The pipe is then laid out as needed and finally filled in with dirt or some other material. Pipelines can also be constructed on land that is not owned by someone else. In this case, they are called "public pipelines" and have certain requirements set by law regarding safety and maintenance. Public pipelines can only be installed by certified contractors who will be responsible for repairing any damage that is done to the pipeline. Finally, pipelines can be buried less than two feet deep under your own property without permission if they are used exclusively for personal use. These private pipelines cannot be inspected by anyone except you and they must be shut off at both ends. They are also required by law to be removed when you build another structure (house, garage, etc.) on your property. In conclusion, pipelines are built just like any other piece of infrastructure such as bridges or roads, using materials that are safe for transport of oil and gas. Their construction requires special skills that only certain companies can afford because of the expensive equipment needed. Also, notice that no pipeline can be built inside a city unless it has been approved by officials there.