A correction for page 14
The printed book says Olympus Mons is 25 miles tall. That is a mix-up between two units: 25 is the number in kilometres. In miles it is about 16.
Sorry about that one. It is fixed in the next printing.
Found 30 October 2026
How do we know?
↻ One of the pages with a moving picture
Nobody has ever climbed it. We measured it with a laser, from space.
A spacecraft called Mars Global Surveyor carried an instrument called MOLA — a Mars Orbiter Laser Altimeter. Ten times every second, it fired a pulse of light at the ground below and timed how long the light took to bounce back up.
Light always travels at the same speed, so the time tells you the distance. Do that about 600 million times from orbit, and you end up with a height map of an entire planet.
MOLA took those measurements between 1997 and 2001. That map is still the one scientists use today.
Wow number
600 million pulses of laser light that MOLA bounced off the surface of Mars to build its height map.
Where in the world?
Olympus Mons, Mars on the Tharsis bulge, west of the Valles Marineris canyons — 18.65° N, 133.8° W.
Where these facts come from
Every number on page 14, and who measured it
Olympus Mons is about 25 km (16 mi) tall
Mars Fact Sheet — NASA Goddard Space Flight Center
nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html
We checked it on 30 October 2026
Olympus Mons is a shield volcano about 600 km (370 mi) across, roughly the size of the state of Arizona, with flank slopes of about 5 degrees — so the climb from the edge to the summit is about 180 mi
Mars — NASA Science
science.nasa.gov/mars
We checked it on 30 October 2026
The name, position and 600 km diameter of Olympus Mons
Gazetteer of Planetary Nomenclature — International Astronomical Union and the USGS
planetarynames.wr.usgs.gov
We checked it on 30 October 2026
MOLA fired 10 laser pulses a second and took about 600 million measurements between 1997 and 2001
Mars Global Surveyor — NASA
science.nasa.gov/mission/mars-global-surveyor
We checked it on 30 October 2026
Mount Everest is 8,848.86 m tall — 8.85 km, or 5.5 mi
Joint announcement of the height of Mount Everest — Nepal and China
Published 8 December 2020. We checked it on 30 October 2026
Airliners cruise at about 11 km (7 mi)
Flight levels and cruising altitudes — Federal Aviation Administration
faa.gov
We checked it on 30 October 2026
Has this changed?
Olympus Mons is not going anywhere, so this number will stay where it is.
But some numbers in this book do change. Page 28 asks how many glaciers are left in Glacier National Park, and that number keeps dropping. Pages like that one show you today’s number right here, not the number from the day the book was printed. If you see a little clock beside a number in the book, come back and check it.
Words from this page
- volcano
- An opening in a planet’s surface where melted rock comes out.
- shield volcano
- A wide volcano with gentle sides, built up from runny lava — shaped like a warrior’s shield lying flat on the ground.
- altimeter
- A tool that measures height. MOLA’s altimeter used a laser.
Try it!
Olympus Mons is a shield volcano: wide, with gentle sides, built up out of runny lava. It is so gentle that if you were standing on it you could not tell you were on a mountain at all. Its slopes tilt about 5 degrees — about the same as a wheelchair ramp.
Find a ramp near you and walk up it. Now imagine walking up that same gentle slope for 180 miles. That is the climb to the top.
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