"The world is always full of the sound of waves. The little fishes, abandoming themselves to the waves, dance and sing, and play, but who knows the heart of the sea, a hundred feet down? Who knows its depth?"
- From the book Musashi by Eiji Yoshikawa
Miyamoto Musashi was a Japanese warrior, philosopher, and writer who lived about 400 years ago (1). He was widely acclaimed for his unique two-hand style of combat, which contributed to an undefeated record in 61 duels and ultimately culminated in his status as the most reputable of all Japan's samurai to this day. Many people do not realize that Musashi did not perceive the training of martial skills as the most important components of a warrior. Instead, he maintained the depths of a true warrior were anchored in the training of the mind and the disciplining of the spirit.
Everywhere one looks, one sees the Earth. Yet what is seen is virtually always a part of its surface. It is natural to conceptualize the surface or crust as "Earth" when, at 35 kilometers of depth, it is a mere scratch of the planet's total depth - less than 1% of the 6,000 kilometers of distance separating the Earth's surface from its central core. Most of the Earth's depths are constituted by mantle, a thick layer of rock that makes up 84% of Earth's total volume (2). The core consists of an outer layer, composed largely of fluid iron and nickel, and an inner layer, composed of solid iron and nickel (3).
Earth's depths - crust at the surface, beneath which lies the mantle, outer core, and inner core (3).
Of course, we do not always see the Earth's surface. We can also gaze up at its skies, particularly on a clear, starry night when it is easy to see something of what lies well beyond the Earth, something of realms unknown amongst the stars...and we certainly should look upwards, for this is a good thing to do.
Yet it is harder to see something of what lies beneath the Earth's surface, which also leads to realms unknown. For we should also look inwards...this too is a good thing to do.
The Deepest Caverns
A cavern (cave) is a natural void in the ground, which techinically must be large enough for a human to enter (4). Caverns are formed and developed by the process of speleogenesis, which occurs over millions of years and can involve a combination of chemical processes, water erosion, tectonic forces, atmospheric influences, and even microorganisms. Given the confluence of many of these factors, the diminutive country of Georgia boasts the deepest caverns in the world.
The current record-holder for the title of "world's deepest cavern" is Veryovkina Cavern, which has been surveyed to a depth of 2,212 meters (5). Veryovkina Cavern was chiseled out of the surrounding limestone, dissolved by groundwater and rain running throughout cracks in the rocks over millions of years. Amazingly, up until 2015, the depth of Veryovkina was thought to be a mere 440 meters; only during the last 7 years have its depths become further appreciated. Members of the speleoclub Perovo have been the most instrumental in discovering Veryovkina's depths. However, they have not been its sole explorers. Just last year, a Russian climbing enthusiast named Sergei Kozeev was found dead at a depth of 1,100 meters, hanging by a rope; it is still not clear whether he died from a fall or from the ensuing hypothermia.
Babatunda Pit, near the top of Veryovkina Cavern (5).
Map showing the layout of Krubera Cavern (7).
Since both Veryovkina and Krubera may not yet be fully explored, they may well be deeper than the current records indicate. In fact, the world's truly deepest cavern may not be either of these caverns, but some other cavern, its dark passages never seen by human eyes. Regardless of the depth of the planet's truly deepest cavern, none is likely to ever lay claim to the title of "deepest region on the surface of the planet"...that claim belongs to the oceanic trenches.
The Deepest Trenches
An oceanic trench is a long, narrow depression in the sea floor. Oceanic trenches are formed through plate tectonics, and actually mark the locations where two plates converge (8). The trench develops as one plate subducts beneath the other. Unquestionably, the deepest oceanic trenches in the world are located in the western Pacific Ocean.
The record-holder for the title of "world's deepest trench" is the Mariana Trench south of Japan, which contains a valley, called Challenger Deep, that lies 10,920 meters beneath the waves (9). The pressure at such a depth is over 1,000 times that of the atmosphere at sea level. The Mariana Trench was initially surveyed in 1875 using a weighted rope, which recorded a depth of over 8,000 meters. Only four crewed descents to Challenger Deep have ever occurred, the first of which was achieved by the US oceanographer Don Walsh and fellow Swiss oceanographer Jacques Picard aboard the deep-sea submersible Trieste. An abundance of life exists near the bottom of the Mariana Trench, including "supergiant" amphipods (shrimp-like crustaceans) which are much larger than their shallow-water cousins.
The Mariana Trench was first explored by the deep-sea submersible Trieste (10).
Map showing the location of the Tonga Trench (11).