The interior structure of the Earth, similar to the other terrestrial planets, is layered. These layers can be defined by either their chemical or their rheological properties. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.
Scientific understanding of Earth's internal structure is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravity field of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.
Scientific understanding of Earth's internal structure is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravity field of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.
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The Lost River sucker is a large fish found in the freshwater ecosystems of California and Oregon. It is a long-lived fish, sometimes reaching an age of 40 years or more. Its primary home is in lakes, but it migrates to rivers and streams to breed. The Lost River sucker is threatened by overfishing and by the damming of rivers and the draining of wetlands. 
The Mekong giant catfish is a freshwater fish that grows to be very large — about as large as a grizzly bear in some cases. This enormous fish was historically found throughout the Mekong River, which is the 12th longest river in the world. The waterway spans about 2,983 miles (4,800 kilometers) from its headwaters in the Tibetan Plateau throughout much of Southeast Asia. Currently, Mekong giant catfish populations are in rapid decline, primarily due to dams, overfishing and pollution.
The Russian sturgeon is a large, bony fish that can reach a length of more than 6 feet. It lives in salt water and travels upriver to spawn, which makes it vulnerable to habitat loss on multiple fronts, including hydroelectric dams that create barriers to spawning grounds, pollution in both its freshwater and saltwater homes, and being heavily fished for its meat and eggs, which are used to make caviar. 
Like sharks, skates and rays, sawfish have skeletons made of cartilage rather than bone. The smalltooth sawfish is a long, sleek fish that occurs in shallow coastal and estuarine waters in the Atlantic and Pacific oceans as well as the Gulf of Mexico. It also has a long, flattened snout lined with rows of teeth that it uses to locate and subdue prey. In addition to facing the threat of hunting, pollution and habitat loss, it is extremely vulnerable to being caught in fishing lines.
IUCN Status: Critically Endangered
The brown kiwi is a stocky nocturnal bird that is native to New Zealand. A flightless bird, the brown kiwi feeds by walking slowly through the forest poking its long, slender bill — which is highly sensitive to touch — into the ground in search of worms, insects and larvae. It was once found throughout New Zealand, but is now restricted to fragmented forests and seriously threatened by non-native predators such as pigs, cats and dogs.
IUCN Status: Endangered
The crowned eagle is a mighty bird of prey with broad wings, pale gray plumage and a distinct crest on its head. It occurs throughout central South America in both forest and semi-open areas. Like many birds of prey, the crowned eagle is endangered because of habitat loss. As top predators, they are also vulnerable to toxins present in their prey animals, and are subject to harassment by humans who perceive them as a threat to farm animals. 
