A beaver dam counts as nature, but a mud hut does not
Where does nature stop? By the usual definition, a termite mound or a beaver's dam belongs to the natural world, while a simple mud hut or a solar panel set in empty desert is artificial. The dividing line is not how much a landscape changes, but whether humans did the changing.
Even that line blurs on inspection. No place on Earth is now entirely untouched, so naturalness is better pictured as a sliding scale than a yes-or-no label. Climate change, the loss of species and plastic and chemical pollution reach every corner. Different parts of one landscape can also sit at different points on the scale: a ploughed field has soil minerals much like those of an undisturbed forest floor, but its structure has been thoroughly rearranged.
Some of the least altered places are the hardest to reach. The deep seabed covers more than half the planet's surface and remains among the least modified environments anywhere. Much of the rest has been reshaped around water. Dams create reservoirs and electricity yet block fish from migrating and stop organisms drifting downstream, while clearing forests can bring floods, weaker stream flow and changes to riverside plants. Small streams matter more than their size suggests, acting as corridors that link patches of habitat cut off from each other.
Earth scientists usually divide the natural world into four spheres: rock, water, air and life. Some add ice and soil as spheres of their own. Beneath the rocky crust, the mantle is heated by radioactive decay and, though solid, slowly churns; that motion drags the plates of the surface around. Lakes, by contrast, are fleeting on geological timescales, since each one eventually silts up or breaches its basin.
Overhead, a thin shell of gas held by gravity does a surprising amount of work. Dry air is about 78 percent nitrogen, 21 percent oxygen and 1 percent argon. Ozone, concentrated between roughly 15 and 35 kilometres up, absorbs much of the ultraviolet light that would otherwise damage DNA, and the blanket of air holds warmth overnight, softening the swing between day and night temperatures. Most meteors burn up in the mesosphere, while the International Space Station circles within the thermosphere, still technically inside the atmosphere.
Source: Natural environment
