Which lake is the largest by surface area in the world, covering 371,000 square kilometers (143,000 square miles)? This massive saltwater lake is actually a remnant of a much larger ancient sea called the Paratethys. Located between five countries (Kazakhstan, Russia, Turkmenistan, Azerbaijan, and Iran), it is so large that it contains 40% of all lake water on Earth by volume. It is often called a "sea" because of its size and saline water. Its surface area is larger than Germany. The lake has no outlet – rivers flow into it but none flow out, which is why it is salty. The Volga River provides 80% of its inflow. Since 1960, it has been shrinking due to river diversion for irrigation, losing 40% of its former area.

Which lake is the deepest in the world, reaching a maximum depth of 1,642 meters (5,387 feet)? Located in Siberia, Russia, this ancient lake holds 20% of the world's unfrozen freshwater – more than all the North American Great Lakes combined. It is also the world's oldest lake, estimated to be 25-30 million years old. The lake is so deep that if you stacked the Eiffel Tower or the Washington Monument on top of each other, you would need about five of them to reach the bottom. It is home to over 1,500 species of plants and animals found nowhere else on Earth, including the world's only freshwater seal. The lake lies on a rift valley, similar to those that form ocean basins, and is actually slowly growing wider each year.

Which lake is the highest navigable lake in the world, sitting at 3,812 meters (12,507 feet) above sea level? This South American lake is located in the Andes Mountains on the border of Peru and Bolivia. It is the largest lake in South America by volume and second largest by surface area. According to Inca legend, the sun god Inti was born here, and the lake is considered the birthplace of the Inca civilization. The lake contains 41 islands, some of which are inhabited by indigenous Uros people who live on floating reed islands made of totora reeds. The lake's high elevation means the air has less oxygen, affecting both visitors and local wildlife. It is one of the few lakes where you can get altitude sickness just by being near the shore.

Which lake contains more freshwater than all the North American Great Lakes combined? This Siberian lake holds approximately 23,600 cubic kilometers (5,700 cubic miles) of water – 20% of the world's surface freshwater. It contains 1/5 of all fresh surface water on Earth. The amount of water is so massive that it would take 20% of the world's population a full year to drink it all (about 1.5 trillion bathtubs). Despite its enormous volume, it is only the seventh largest by surface area because of its extreme depth. If the lake were drained, it would take all of Earth's rivers (including the Amazon) about 300 years to refill it.

Which waterfall is the tallest in the world, with a total drop of 979 meters (3,212 feet)? Located in Canaima National Park in Venezuela, this waterfall plunges off a table-top mountain called Auyán-tepui. Its height is so extreme that the water turns into mist before reaching the bottom. The longest single drop is 807 meters (2,648 feet) – nearly 20 times taller than Niagara Falls. It is named after an American aviator who was the first person to fly over it while searching for gold in 1933. The falls are located in a remote jungle and can only be reached by boat and hiking. It inspired the waterfall in the movie "Up" (2009).

Which waterfall has the largest volume of water flowing over it, with an average flow of 1,500 cubic meters (53,000 cubic feet) per second? Located on the Congo River in Africa, this waterfall is actually a series of rapids and cataracts that drop 270 meters (886 feet) over 350 kilometers (220 miles). Unlike most waterfalls that drop vertically, this is a "cataract waterfall" – a long series of rapids. Its flow is so powerful that it drains an area of 3.7 million square kilometers, nearly the size of the entire European Union. At peak flood, the flow can reach 70,000 cubic meters per second – enough to fill 28 Olympic swimming pools every second. Only the Amazon River has a larger flow volume.

Which lake has the highest salinity of any permanent lake, with salinity 10 times higher than the ocean? Located in Ethiopia, this lake has no outlet and sits in one of the hottest places on Earth (average temperature 34°C / 93°F). Its salinity is about 35% compared to the ocean's 3.5%. The lake is actually a volcanic crater that was formed by volcanic activity 10,000 years ago. The water is so salty that it feels oily to the touch, and you can float effortlessly like in the Dead Sea – but even more buoyant. The surrounding area is one of the most inhospitable places on Earth, with salt extraction being the only economic activity. The lake's bright colors (green, yellow, red) come from thermophilic (heat-loving) bacteria and algae.

Which lake is considered the oldest in the world, estimated to be 25-30 million years old? This ancient Siberian lake began forming when a rift valley pulled apart, similar to how the Red Sea is forming today. Most lakes exist for only 10,000 to 100,000 years before they fill with sediment or drain. This lake has survived for millions of years because it is located on an active rift zone that continues to widen. The lake's great age has allowed unique species to evolve in isolation – over 80% of its plant and animal species are endemic (found nowhere else). Scientists study sediment cores from its bottom to learn about climate changes going back millions of years.

Which waterfall is the widest in the world, stretching 1.7 kilometers (1.1 miles) across? Located on the border of Zambia and Zimbabwe, this African waterfall is locally called "Mosi-oa-Tunya" which means "The Smoke That Thunders." It is 108 meters (354 feet) tall and 1,708 meters (5,604 feet) wide. The spray from the falls can be seen from 50 kilometers (30 miles) away, and the mist creates a permanent rainbow. The falls have two islands on the crest that divide the water into several channels. During the rainy season (April-May), over 500 million cubic meters of water flow over the falls every minute. The water then flows through a series of gorges, creating spectacular whitewater rapids.

Which lake, located on Manitoulin Island in Lake Huron, is the largest lake on an island in a lake? This is a lake that forms a "lake island lake" – a lake located on an island that is itself inside another lake. Manitoulin Island is the world's largest freshwater island (2,766 sq km), located in Lake Huron (one of North America's Great Lakes). The lake on the island is 77 square kilometers (30 square miles). It even has its own islands, creating a lake-within-an-island-within-a-lake geographic oddity. This phenomenon is called a "recursive lake" or "lacustrine island." There are also several smaller lakes on the same island. The region is popular for fishing, boating, and exploring unique geological features.