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How long is 668 kilometers?

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It's about one-tenth as long as The Amazon River.
In other words, 668 kilometers is 0.10380 times the length of The Amazon River, and the length of The Amazon River is 9.6340 times that amount.
(from Nevado Mismi, Arequipa, Peru through near Marajó, Brazil) (approximate distance)
The Amazon River has an approximate distance of 6,437 km. Although by most measures it is the largest and one of the most significant rivers in the world, the region surrounding the Amazon is mostly sparsely populated and the river itself has no bridges across it at any point.
It's about one-fifteenth as long as The Great Wall of China (total).
In other words, the length of The Great Wall of China (total) is 13.20 times 668 kilometers.
(长城, 長城, Chángchéng, a.k.a. 万里长城, 萬里長城, Wànlǐ Chángchéng) (a.k.a. "The long wall of 10,000 Li") (from Shanhaiguan through Lop Nur, China) (total length, all branches)
The Great Wall of China, including all branches and trenches, is 8,850 km. Built and maintained in multiple sections over about eleven centuries, the Great Wall is currently suffering the effects of erosion, especially in the older sections made primarily out of mud.
It's about one-twentieth as long as The Diameter of Earth.
In other words, 668 kilometers is 0.052366392 times the length of The Diameter of Earth, and the length of The Diameter of Earth is 19.096217 times that amount.
(Equatorial)
The Earth — not a perfect sphere, but rather an oblate spheroid with bulged middle — has a diameter of approximately 12,756.2730 kilometers at the Equator. The first complete view of Earth's diameter was in a photograph taken from a V-2 rocket launched in 1942 by the United States Army, which reached an altitude of 104.607360 kilometers.
It's about thirty times as tall as Olympus Mons.
In other words, the height of Olympus Mons is 0.04 times 668 kilometers.
(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 27 km. The mountain has been known to astronomers since the nineteenth century because it is tall enough to rise above Mars' frequent dust storms.
It's about one-sixtieth as long as The Circumfrence of Earth.
In other words, 668 kilometers is 0.01666874 times the length of The Circumfrence of Earth, and the length of The Circumfrence of Earth is 59.9925370 times that amount.
(Equatorial)
The Earth — not a perfect sphere, but rather an oblate spheroid with bulged middle — has an Equatorial circumference of approximately 40,075.014 kilometers. The first airplane trip around the circumference of the Earth (with a few stops along the way), was completed in 1924 by the United States Army Air Service, and took 175 days.
It's about 60 times as tall as Maxwell Montes.
In other words, 668 kilometers is 61 times the height of Maxwell Montes, and the height of Maxwell Montes is 0.016 times that amount.
(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 11 km. Since Venus does not seem to have the kind of tectonic activity which gives rise to mountains on Earth, the origin of the Venusian mountain remains the subject of some dispute.
It's about 60 times as tall as The Challenger Deep (Marianas Trench).
In other words, 668 kilometers is 61.2730 times the height of The Challenger Deep (Marianas Trench), and the height of The Challenger Deep (Marianas Trench) is 0.01632 times that amount.
(near Marianas Islands, a.k.a. Ladrones Islands, northwestern Pacific Ocean) (depth below sea level)
The Challenger Deep, at the southern end of the Marianas Trench, reaches a depth of 10.902 km below sea level. There has been only one manned expedition to the bottom of Challenger Deep — a 1960 voyage that took 4 hours and 48 minutes to reach the bottom.