The Answer
20,000 leagues is about 72,000 nautical miles. The title 20,000 Leagues Under the Sea is not actually referring to a journey to a depth of 20,000 leagues. Instead it is actually referring to a journey spanning a horizontal distance traveled ‘under the sea’.
I have to say, that without thinking about it very much, I always assumed that the Jules Verne novel 20,000 Leagues Under the Sea was referring to a journey to a depth of 20,000 leagues. In one part of my mind, I was aware that a league was approximated by something like the distance you can travel in an hour, but I thought perhaps a depth league was different than a distance league, the way a nautical mile and a statute mile are different.
In fact, the title is referring to the horizontal distance traveled by Captain Nemo and his companions travelled in their underwater vessel – the Nautilus.
If you want to avoid confusion, perhaps it’s best to look at the original French title, 20,000 Lieues Sous les Mers—20,000 Leagues Under the Seas. That plural, “seas,” makes it clear that Verne’s writing about a trip through the world’s seas, not into their darkest depths.
So how far is a league?
A league is usually thought of as the distance a person could walk in an hour — 3 miles. However, that’s a land league. A marine league would be 3 nautical miles.
According to Webster’s 1913 Dictionary – “The marine league of England and the United States is equal to three marine, or geographical, miles of 6080 feet each.” (per British-American System of Units – The Physics Hypertextbook).
20,000 (nautical) leagues then would be about 72,000 nautical miles (116,000km) or 60,000 statute miles.
What if it was depth?
The deepest part of the ocean is about 7 miles. In order to travel to a depth of 72,000 miles in the ocean, the earth would need to be roughly 10,000 times bigger (wider) than it currently is. To put that in context, Jupiter is only 11 times wider than the earth and the sun is only 109 times wider.
So, a proportionally scaled earth would need to be 91 times as large as the sun.
I’ll leave it to an intrepid reader to figure out what the pressure would be at a depth of 72,000 miles.
Addendum – Even if it is horizontal distance…
My son raised a valid point upon hearing about this. He asked “Isn’t that more than the circumference of the earth?” It turns out that it is, in fact, considerably more. At its widest point (the equator) the earth is 24,901 miles around (40,075 km). Those are statute miles. So a journey of 20,000 leagues or 60,000 statute miles, would take you around the earth nearly two and a half times. That is some voyage.
How long would that take?
The record speed for a submarine is around 33 knots or roughly 38 mph. However, I’m not going to use that measure. Instead, I’m going to use the top speed of the first nuclear submarine – the USS Nautilus – which is 23 knots or roughly 26.5 mph. To go full speed for 60,000 miles, it would take 2264 hours or roughly 94 days, which is much less than I had imagined (although I’m sure the Nautilus would have been slower than the Nautilus).
Incidentally, at that speed, you could go around the world in much less than 80 days.