The Veluwemeer Aqueduct connects the mainland of Harderwijk with Flevoland, the largest artificial island in the word. It’s 82ft. long, 62ft.wide, 9ft. deep and was completed in 2002. Small boats can pass through it and there are footpaths located on either side for pedestrians.
The “Well to Hell” is an urban legend regarding a putative borehole in Russia which was purportedly drilled so deep that it broke through into Hell. It’s now completely abandoned. But when you look closely through map, it’s nothing enormous at all. According to comments in google maps, the borehole is only 9 inch wide.