SIGHT: Liquid pools, continuous bubbling in wet or flooded areas, an oily sheet on water surfaces, vaporous fogs and/or blowing dirt around a pipeline area.  Dead or discolored plants in an otherwise healthy area of vegetation or frozen ground in warm weather are all signs of a pipeline leak. Natural gas is colorless, tasteless, and odorless, but vapor and "ground frosting" may be visible at high pressures. A natural gas leak may also be indicated by dust blowing from a hole in the ground or flames, if the leak is ignited.

SOUND: Volume can range from a quiet hissing to a loud roar, depending on the size of the leak and pipeline system. 

SMELL: An unusual smell, petroleum odor, or gaseous odor will sometimes accompany pipeline leaks. Natural gas and HVLs are colorless, tasteless, and odorless, unless commercial odorants or Mercaptan is added. Gas transmission/gas gathering pipelines are odorless, but may have a hydrocarbon smell.