It's about one-five-hundred-fiftieth as tall as Maxwell Montes
In other words, 21 meters is 0.002 times the height of Maxwell Montes, and the height of Maxwell Montes is 530 times that amount.(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 11,000 meters. 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-one-thousand-five-hundredth as tall as Olympus Mons
In other words, 21 meters is 0.0008 times the height of Olympus Mons, and the height of Olympus Mons is 1,300 times that amount.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 27,000 meters. 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 1,500 times as long as an Aspirin
In other words, 21 meters is 1,500 times the length of an Aspirin, and the length of an Aspirin is 0.0007 times that amount.(for up & up aspirin, 325mg, regular strength tablets) (a.k.a. acetylsalicylic acid, a.k.a. ASA)
A single aspirin tablet measures 0.014 meters in diameter. One of the most common medicines in the world and a foundation of worldwide health systems, an estimated 40,000 metric tons of aspirin are consumed each year.
It's about one-four-thousandth as long as The Panama Canal
In other words, 21 meters is 0.0002 times the length of The Panama Canal, and the length of The Panama Canal is 4,200 times that amount.(Canal de Panamá) (Panama)
Connecting the Atlantic and Pacific Oceans across the Isthmus of Panama, the Canal measures 88,000 meters in length. Ships passing through the Canal are raised (and then lowered) 26 meters above sea level through the lock-and-dam system.
It's about one-five-thousandth as wide as The English Channel
In other words, 21 meters is 0.00020 times the width of The English Channel, and the width of The English Channel is 5,100 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 107,000 meters, 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-five-thousand-five-hundredth as long as The 405
In other words, 21 meters is 0.000180 times the length of The 405, and the length of The 405 is 5,549.4 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 116,540 meters. The 405 is the most heavily-trafficed freeway in the United States, with an average speed as low as 8 kph during rush hours.
It's about one-five-thousand-five-hundredth as long as Hadrian's Wall
In other words, 21 meters is 0.0002 times the length of Hadrian's Wall, and the length of Hadrian's Wall is 5,600 times that amount.(a.k.a. Roman Wall, a.k.a. The Wall, a.k.a. Vallo di Adriano, a.k.a. Vallum Aelium, a.k.a. Severus' Wall) (from North Tyneside, Tyne and Wear, England, United Kingdom to Cumbria, North West England, United Kingdom)
Hadrian's Wall, which crossed England form the North Sea to the Irish Sea during the time of Roman rule, measures 120,000 meters (about 79 Roman miles). Roman soldiers — at one time up to 10,000 of them — only occupied Great Britain for about 246 years after the wall was built, after which time the local Brits began to take residence the wall's garrisons.