14 - Under The Sea: Underwater Waterfall

Just off the coast of the island of Mauritius in Africa, there is an amazing optical illusion that is a sight to behold. Here, it appears as though there is a waterfall that runs under the water.

This optical illusion is caused by the ocean currents that pull sand and silt in a certain direction, making it look as though it is falling down. The seemingly large drop is actually no deeper than the rest of the surrounding water. The underwater waterfall can only be seen from the air, and pictures have been captures by planes, helicopters, and even drones!

The colour of the ground changes depending on what kind of sand and silt is underneath of it. This causes the ground beneath to appear as though it has different depths when it doesn't.

13 - Natural Architecture: Living Bridges

In the Northeast Indian state of Meghalaya, a strange sort of bridge exists. These "living root bridges" are a form of tree shaping that are made from the aerial roots of rubber trees.

Though these bridges are created with the guidance of the Khasi and Jaintia people, they are strong enough to support about 50 people at a time. The roots grow and strengthen over time. It takes approximately 20 years for the bridge to be strong enough to hold people.

A root bridge can last for hundreds of years as long as the tree remains healthy. The bridge will naturally self-renew and self-strengthen as its roots grow thicker. Unlike an artificial structure, these living bridges are able to withstand the high level of soil erosion that occurs by monsoon rains. Living material is also much more resistant to destruction by termites, and animals as it is able to renew itself if damage is inflicted.

In order to make the rubber tree's roots grow in the right direction, several different strategies are used. Sometimes the roots of the trees are pulled together, tied, and twisted by hand for them to merge together over time. This is the most common method.

Less common ways include creating scaffolds out of wood and bamboo, and then training the young roots of the rubber tree to go out across them.

In some places, betel nut trunks are sliced down the middle and hollowed out to create root-guidance systems. This gives the roots an advantage, as they get nutrients from the trunk as it decays.

12 - Breaking Records: Son Doong Cave

Son Doong Cave, located in Quang Binh Province, Vietnam, is the largest cave in the world. It covers 3,840,000 cubic meters. It is more than five kilometers long, 200 meters high, and 150 meters wide.

Its name means "Cave of the Mountain River" in Vietnamese. It was formed in soluble limestone, making it a solutional cave.

The cave was discovered by a local man named Ho Khanh in 1991 when he was out hunting. However, local people did not enter the cave due to the whistling sound of wind and the roar of a rushing stream in the cave making terrifying sounds. Only in 2009 did a group of cavers enter.

The cave contains some of the tallest known stalagmites in the world, reaching up to 70 meters tall. It consists of two dolines (areas with collapsed ceilings). This allows sunlight to enter the cave so that foliage can grow. This makes it home to many animals, including monkeys, flying foxes, and more. The cave has its own climate suitable for plant and algae growth.

Reaching the cave requires two days of intense jungle trekking and river crossings. Tours to the cave have been open to the public since 2013.