10 most difficult Earth riddles

Iron is most abundant in the Earth's outer and inner core, and it is the fourth most common element in the Earth's crust.

Zhytomyr is a city in the north of the western half of Ukraine.The Teteriv river generally forms the southern boundary of Zhytomyr, though there are also some small areas of Zhytomyr city territory below the southern bank of the river. Its northern part is contaminated by the Chernobyl power plant.

Kiribati, is an island republic in the Central Pacific. It comprises 33 coral atolls and isles stretching along the equator. It is the only country in all four hemispheres. The country's capital is South Tarawa. It is pronounced "Kiri-bahs"

For each circle, the length of the circumference is c=2πr. Hence, if a radius grows by 1 meter, the circumference grows by 2 π meters - no matter what the initial value of radius was.

Henry Cavendish, british physicist (1731-1810) was the first scientist to calculate mass of the Earth. Greater accuracy has not been achieved for nearly 100 years. He only made a mistake about 1% compared to today's calculations. Cavendish estimated the mass of the Earth to be 6 billion trillion tons. Modern calculations say about 5.9725 billion trillion tons. (5,97219×10^24 kg)

Pluto, our tiny intrasolar neighbor, has a surface area of only a scant 6.45 million square miles. That might seem like quite a few square miles to sling around, but by comparison, that’s only one-third of one percent of the Earth’s total surface and small enough to fit entirely within the borders of Russia (6.6 million square miles). Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered and is the largest known plutoid (or ice dwarf).

With a salinity level of over 40%, Don Juan Pond is the saltiest of the Antarctic lakes. This salinity allows the pond to remain liquid even at temperatures as low as −50 °C (−58 °F) due to the interference of salts with the bonding of water molecules. Don Juan Pond was discovered in 1961 by George H. Meyer.

Zircons from the Western Australian Jack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth.

The last universal common ancestor (LUCA) is the most recent population of organisms from which all organisms now living on Earth have a common descent. LUCA is not thought to be the first living organism on Earth, but only one of many early organisms, whereas the others became extinct. In 2016 study identified a set of 355 genes inferred to have been present in LUCA. LUCA is estimated to have lived some 3.5 to 3.8 billion years ago in the Paleoarchean era.

Geography is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the word γεωγραφία was Eratosthenes (276–194 BC). Geography is an all-encompassing discipline that seeks an understanding of Earth and its human and natural complexities—not merely where objects are, but also how they have changed and come to be.