Japanese has three separate writing systems used simultaneously — hiragana, katakana, and kanji.

The Earth's crust varies in thickness beneath continents and oceans.

The Arctic Circle experiences extreme variations in daylight.

The eradication of polio in many countries marked a major health milestone.

The lyrebird of Australia can mimic almost any sound — including chainsaws, camera shutters, and car alarms.

The first printed book was the Gutenberg Bible, produced around 1455 in Mainz, Germany.

The entire population of Earth in 1 AD was around 300 million — roughly equal to the current population of the United States.

China and India together contain over one-third of humanity.

The ocean contains structures larger than mountain ranges.