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Earth is unique in our solar system as it has liquid water in oceans, rivers and lakes etc. Other planets are either too hot or too cold for liquid water which is necessary for life forms as they now exist on Earth.

QUESTION: How do we know what structure is UNDER Earth’s crust? ANSWER: Most knowledge of the Earth’s interior comes from the study of seismic waves - shock waves that pass through the planet after a quake or explosion. There are three different kinds of waves:

P	P (primary) waves can travel through all parts of the Earth. They reach the side of the globe opposite a shock’s focus in 20 minutes.
S	S (secondary) waves cannot go through liquid so they leave a circular shadow that shows where they were blocked by the Earth’s molten core.
	P-waves and S-waves bend as they pass through the Earth, refracted by density changes in the material. Refraction in the core causes a ring-shaped shadow where no P-waves can be detected.
L	L-waves travel in the Earth’s surface near a focus, causing the ground to shear from side to side or move up and down.

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