The 2 km walk deep under Paris through the Catacombs Ossuary where 6 million human remains are interred in public view.

(Hint: If you want to shoot video here, I'd recommend the GoPro Hero 6, which has much better low light performance than the old gear used here. http://amzn.to/2Am1Gxv Bring a headlamp!)

From Wikipedia:
The Catacombs of Paris or Catacombes de Paris are an underground ossuary in Paris, France. Located south of the former city gate (the "Barrière d'Enfer" at today's Place Denfert-Rochereau), the ossuary holds the remains of about 6 million people[1] and fills a renovated section of caverns and tunnels that are the remains of Paris' stone mines. Opened in the late 18th century, the underground cemetery became a tourist attraction on a small scale from the early 19th century, and has been open to the public on a regular basis from 1867. Following an incident of vandalism, they were closed to the public in September 2009 and reopened 19 December of the same year.[2]

The Catacombs entry is in the western pavilion of Paris's former Barrière d'Enfer city gate. After descending a narrow spiral stone stairwell of 19 meters to the darkness and silence broken only by the gurgling of a hidden aqueduct channelling local sources away from the area, and after passing through a long (about 1.5 km) and twisting hallway of mortared stone, visitors find themselves before a sculpture that existed from a time before this part of the mines became an ossuary, a model of France's Port-Mahon fortress created by a former Quarry Inspector. Soon after, they would find themselves before a stone portal, the ossuary entry, with the inscription Arrête, c'est ici l'empire de la Mort ('Stop, this is the empire of Death').

Beyond begin the halls and caverns of walls of carefully arranged bones. Some of the arrangements are almost artistic in nature, such as a heart-shaped outline in one wall formed with skulls embedded in surrounding tibias; another is a round room whose central pillar is also a carefully created 'keg' bone arrangement. Along the way one would find other 'monuments' created in the years before catacomb renovations, such as a source-gathering fountain baptised "La Samaritaine" because of later-added engravings. There are also rusty gates blocking passages leading to other 'unvisitable' parts of the catacombs -- many of these are either un-renovated or were too un-navigable for regular tours.

In a cavern just before the exit stairway leading to a building on the rue Dareau (former 'rue des Catacombes') above, one could see an example of the Quarry Inspection's work in the rest of Paris' underground caverns: its roof is two 11-metre high domes of naturally degraded, but reinforced, rock; the dates painted into the highest point of each bear witness to what year the work to the collapsing cavern ceiling was done, and whether it has degraded since. These "fontis" were the reason for a general panic in late-18th-century Paris, after several houses and roadways collapsed into previously unknown caverns below.