Trip to the Center of the Earth

Here's your nightly math! Just 5 quick minutes of number fun for kids and parents at home. Read a cool fun fact, followed by math riddles at different levels so everyone can jump in. Your kids will love you for it.

Trip to the Center of the Earth

November 24, 2015

What happens if you dig a hole straight down into the ground? How far do you have to dig to reach the bottom? That’s what Bedtime Math fan Ajax wants to know. Well, it depends what you mean by the “bottom.” If you just want to dig to the center of the Earth, it’s 3,958 miles to the very middle of that ball. But if you want to dig as far as you can until you show up on the other side, it will be twice as far, or 7,916 miles. The crazy thing is, the deepest humans have ever dug is just 7 1/2 miles, or less than 1/1000th of the way across! By the way, there’s a website called FreeMapTools where you mark your starting point, and it will show you the spot exactly on the other side of Earth where you would pop out. Just remember that if you do try this, the center of Earth is a 10,000-degree F blob of iron and nickel, so you’ll need your heat suit.

Wee ones: If you dig 7 miles down, then dig 1 more mile to beat the record, how far do you dig?

Little kids: Which is cooler, 1 thousand degrees or 10 thousand degrees?  Bonus: Earth is 7,916 miles across. Close your eyes and see if you can remember that number!

Big kids: Who digs farther, you if you dig down 6 miles a day for 6 days, or your friend who digs 5 miles a day for a whole week?  Bonus: If you dig exactly 20 miles every day, by how much will you overshoot the exact middle spot at 3,958?

The sky’s the limit: If you dig 10 miles the first day, 10 miles again the 2nd day, then 20 the 3rd day, then 30, then 50… how many miles do you think you dig the day after that?




Wee ones: 8 miles.

Little kids: 1 thousand degrees.  Bonus: 7,916…7,916….see how long you can remember that.

Big kids: You dig farther, at 36 miles vs. your friend’s 35. By the way that happens with any number multiplied by itself (squared): the answer will be 1 more than multiplying that number plus 1 by that number minus 1. Try it!  Bonus: Just 2 miles, since 3,960 will be a multiple of 20.

The sky’s the limit: 80 miles. If you noticed the pattern, each ay you dig as far as the previous two days added together. It’s the famous Fibonacci series multiplied by 10, where the Fibonacci numbers are 1, 1, 2, 3, 5, 8, 13…

And thank you Ajax for digging into math with us!

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About the Author

Laura Overdeck

Laura Overdeck

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking while still in diapers, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.

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