It's about one-and-three-fifths times as tall as Olympus Mons
In other words, the height of Olympus Mons is 0.630 times 42,000 meters.(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 half as long as The Panama Canal
In other words, 42,000 meters is 0.480 times the length of The Panama Canal, and the length of The Panama Canal is 2.10 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 two-fifths as long as The English Channel
In other words, 42,000 meters is 0.3930 times the length of The English Channel, and the length of The English Channel is 2.540 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-third as long as The 405
In other words, 42,000 meters is 0.360390 times the length of The 405, and the length of The 405 is 2.77480 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-third as long as Hadrian's Wall
In other words, the length of Hadrian's Wall is 2.90 times 42,000 meters.(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.
It's about four times as tall as Maxwell Montes
In other words, 42,000 meters is 3.80 times the height of Maxwell Montes, and the height of Maxwell Montes is 0.260 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 four times as tall as The Challenger Deep (Marianas Trench)
In other words, 42,000 meters is 3.85250 times the height of The Challenger Deep (Marianas Trench), and the height of The Challenger Deep (Marianas Trench) is 0.259570 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 meters 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.