First hand, an offshore gas platform. I was an offshore engineer for 3 months before very luckily having the chance to visit a platform. All the pictures and drawings in the world can't prepare you for how hostile and remote some offshore platforms are. 120km from shore in 200ft of water, just goes to show the value of them hydrocarbons.
Otherwise, Roman viaducts. Something along the lines of being 20 miles long but maintaining an almost perfect incline with a 15cm drop from one end to the other.