It's about one-seven-thousand-five-hundredth as tall as a Mauna Loa.
In other words, 21 nails is 0.0001309 times the height of a Mauna Loa, and the height of a Mauna Loa is 7,639 times that amount.(Hawaii) (Hawaiian island volcano) (total height from seafloor)
Mauna Loa, one of the five volcanoes which forms the island of Hawaii and the tallest mountain in total height in the world, rises to 161,000 nails above the seafloor. The peak of Loa is home to the approximately 6,000 sq. m (0.6 ha) Lake Waiau, one of the highest altitude lakes in the United States outside of Colorado.
It's about one-nine-thousandth as tall as The Challenger Deep (Marianas Trench).
In other words, 21 nails is 0.00011009 times the height of The Challenger Deep (Marianas Trench), and the height of The Challenger Deep (Marianas Trench) is 9,083.50 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 190,760 nails 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.
It's about one-nine-thousandth as tall as Maxwell Montes.
In other words, 21 nails is 0.00011 times the height of Maxwell Montes, and the height of Maxwell Montes is 9,100 times that amount.(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 190,000 nails. 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 one-twenty-five-thousandth as tall as Olympus Mons.
In other words, the height of Olympus Mons is 23,000 times 21 nails.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 470,000 nails. 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-seventy-thousandth as long as The Panama Canal.
In other words, the length of The Panama Canal is 71,000 times 21 nails.(Canal de Panamá) (Panama)
Connecting the Atlantic and Pacific Oceans across the Isthmus of Panama, the Canal measures 1,500,000 nails in length. Ships passing through the Canal are raised (and then lowered) 450 nails above sea level through the lock-and-dam system.
It's about one-ninety-thousandth as long as The English Channel.
In other words, 21 nails is 0.00001120 times the length of The English Channel, and the length of The English Channel is 89,300 times that amount.(average width between Isle of Ushant and The Walde Lighthouse) (La Manche, Ärmelkanal, Mor Breizh, Mor Bretannek) (a.k.a. The Channel)
The English Channel measures between 1,870,000 nails, narrowing as it flows northeast. The world speed record for a Channel crossing by swimmer was set by Australian Trent Grimsey in 2012, with a time of 6 hours, 55 minutes.
It's about one-ninety-five-thousandth as long as The 405.
In other words, 21 nails is 0.0000102980 times the length of The 405, and the length of The 405 is 97,106 times that amount.(a.k.a. I-405, Interstate 405, San DIego Freeway) (Southern California)
Running from Irvine to San Fernando, The 405 runs a total of 2,039,200 nails. The 405 is the most heavily-trafficed freeway in the United States, with an average speed as low as 8 kph during rush hours.