EARTH (MEDIA GENERAL) – In honor of Earth Day, here are five interesting facts about “the blue marble.”

  1. It’s all in the name

Earth is the only planet in our solar system not named for a Roman god or goddess. Mercury, Venus, Mars, Jupiter and Saturn were named during ancient times, as they were visible to the human eye. For the sake of consistency, Uranus, Neptune and Pluto were named in a similar manner when they were discovered in 1781, 1846 and 1930, respectively.

Earth is derived from the Old English word “ertha” and the Anglo-Saxon word “erda,” which means ground or soil.

  1. The Earth is slowing down

The Earth’s rotation is slowing, but at an extremely slow pace. The Earth’s rotation is approximately 17 milliseconds longer per 100 years – meaning in 140 million years, one full rotation will take 25 hours instead of 24.

  1. An unblemished surface

Unlike most rocky planets that get hammered with meteorites and other space junk, Earth’s surface has relatively few visible impact craters. Scientists credit this to the Earth’s active geology with processes like tectonic shifts and erosion that continually reshape the planet’s surface.

  1. Challenger Deep

Most fifth-graders know Mount Everest is the highest point on Earth — 29,029 feet above sea level — but do you know the lowest point on Earth? That title belongs to Challenger Deep, a small depression in the bottom of the Mariana Trench in the Pacific Ocean. Located about 189 miles southwest of Guam, Challenger Deep is measured variably between 35,755 and 35,814 feet deep.

Only four descents – two manned, two unmanned – have ever made it to the bottom of Challenger Deep. The first descent was by Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Don Walsh aboard Trieste in 1960.

  1. Water trapped inside the Earth’s surface

Recently, scientists have discovered a vast reservoir of water trapped below the Earth’s surface that brings new hypotheses about how our planet formed over time. Icy comets striking Earth used to be the prevailing theory of the origin of Earth’s water.

Researchers say water is locked up in a mineral called ringwoodite about 400 miles beneath the Earth’s crust. Scientists believe there is enough water below the surface to fill the Earth’s oceans at least three times.

“Geological processes on the Earth’s surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of sight,” Geophysicist Steve Jacobsen told The Guardian in 2014.

“I think we are finally seeing evidence for a whole Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades.”