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Classroom guide: dig through the Earth

Free lesson ideas that use this site to teach map coordinates, Earth’s layers and a little physics. No sign-up, no download. It works on any computer or tablet with a web browser, including school Chromebooks with graphics acceleration turned off.

Grades 3–8 · 20–30 minutes

1. Find your antipode

Students will: read latitude and longitude, use the north/south and east/west hemispheres, and discover that most of the opposite side of the world is ocean.

Ask the class to guess: “If we dug straight down, where would we come out?” Write down the guesses (China is the classic answer).
Open the map and search for your school’s address. The site shows where the tunnel comes out and how far it is to the nearest land.
Work it out by hand: flip the latitude (north ↔ south) and subtract the longitude from 180°, switching east ↔ west. Check against the map.
Discuss: why do almost all tunnels end in the ocean? (About 71% of Earth is covered by water, and the continents mostly don’t sit opposite each other. Only about 13% of land has land on its other side.)

Extend it: Have small groups pick a country from the countries list and report back where it lands. Who found a land-to-land match?

Connects to NGSS 4-ESS2-2 (patterns in maps of Earth’s features) and grade-level geography standards on latitude and longitude.

Grades 3–8 · 20–40 minutes

2. A trip through Earth’s layers

Students will: name Earth’s layers in order, compare their thickness, temperature and state (solid or liquid), and explain how we know about them without going there.

Pick a starting point on the map, then press Kids: Dig with Digby!. The dig stops at every layer with a short lesson card and a quick question; students press “Keep digging!” to move on. It works well on a projector, with the class voting on each question.
Switch to the Globe view and click each layer of the cutaway for its explainer card.
Have students fill in the layer table on the worksheet as they go.
Read How do we know what’s inside the Earth? together and talk about evidence: earthquake waves, meteorites and lab experiments.

Make a model: On a long strip of paper, draw the layers to scale with 1 cm = 100 km (the radius is about 64 cm). The crust is only a few millimeters thick!

Connects to NGSS MS-ESS2-1 (cycling of Earth’s materials and the energy that drives it).

Grades 6–10 · 30–45 minutes

3. Tunnel math: shorter than walking

Students will: compare arc length and chord length on a circle, and use the Pythagorean theorem to find how deep a straight tunnel goes.

On the map, set Dig from to your town and Dig to a city in another country. The panel shows the tunnel length, the walking distance and the deepest point.
For a circle of radius R = 6,371 km and an angle θ between two cities, the walking distance is R·θ (θ in radians) and the tunnel length is 2R·sin(θ/2). Have students check the site’s numbers.
The deepest point of the tunnel is at its middle. Its distance from the center is R·cos(θ/2), so the depth is R − R·cos(θ/2). Draw the right triangle that shows why.
Which layers does the tunnel pass through? Compare with the layer depths: crust to 35 km, upper mantle to 660 km, lower mantle to 2,890 km, outer core to 5,150 km.

Challenge: How far apart must two cities be for a straight tunnel between them to reach the core (deeper than 2,890 km)? (Answer: an angle of about 114° between them, or roughly 12,650 km of walking.)

Connects to Common Core HSG-SRT.C.8 (right triangles) and HSG-C (circles).

Grades 9–12 · 45–60 minutes

4. The gravity train

Students will: model motion through a tunnel through the Earth as simple harmonic motion and compare a uniform Earth with the real, layered one.

Pose the question: if you jumped into a frictionless tunnel through the center of the Earth, how long until you reached the other side?
For a uniform-density Earth, gravity inside is proportional to distance from the center: g(r) = g·r/R. Students derive simple harmonic motion with period 2π√(R/g) and find the one-way time π√(R/g) ≈ 42 minutes.
Discuss why the real answer is shorter (about 38 minutes): the dense core means gravity stays strong far into the planet.
Surprise: in a uniform Earth, every straight tunnel takes the same 42 minutes, however short. Ask students to explain why. Then use the site to compare real-Earth fall times for short and long tunnels.

Read more: How long would it take to fall through the Earth? and Klotz (2015), “The gravity tunnel in a non-uniform Earth,” American Journal of Physics 83, 231.

Connects to NGSS HS-PS2-4 (gravitational force) and HS-ESS2-3 (models of Earth’s interior).

Worksheet: Dig through the Earth

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🏫 Share worksheet to Google Classroom Answer key (for teachers) →

Name: ______________________________ Date: ______________

Part 1: Where would you come out?

My guess: if I dug straight down from here, I’d come out in ______________________.
My location: latitude ________ ° N / S longitude ________ ° E / W
My antipode (flip the latitude; longitude = 180° − mine, other side): latitude ________ ° ____ longitude ________ ° ____
Using digthru.to, I would actually come out in: ______________________________
The nearest land to my antipode is __________________, about ________ km away.
Why do you think most tunnels through the Earth end in the ocean? ________________________________________________

Part 2: Earth’s layers

Layer	Depth (km)	Solid or liquid?	About how hot?	One cool fact
Crust
Upper mantle
Lower mantle
Outer core
Inner core

Part 3: Think about it

Walking to your antipode is about 20,000 km. Tunneling straight through is about ________ km. Why is the tunnel shorter?
Nobody has ever been to the mantle or core. Name one way scientists know what’s down there: ________________________________
Is the outer core solid or liquid? ____________ How do we know? ____________________________________________

digthru.to · Free to copy for classroom use

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