17 - An Insane Drop: The Cliffs of Moher

Located on the southwestern edge of the Burren region in County Clare, Ireland, the Cliffs of Moher are sea cliffs that run for about 14 kilometers.

The cliffs rise 120 meters above the Atlantic ocean at their southern end at Hag's Head, and reach their maximum height at 214 meters north of O'Brien's Tower.

The Hag's Head is the name given to the most southern point of the cliffs because it looks like a woman's head looking out to sea. One legend states that an old hag named Mal of Malbay fell in love with an Irish hero, Cu Chulainn. Cu was chased across Ireland, and escaped by hopping across sea stacks as if they were stepping stones. Mal, however, lost her footing and was dashed against the cliff.

The cliffs consist mainly of Namurian shale and sandstone. The oldest rocks are at the bottom of the cliffs.

At peak season, it is possible to see approximately 30,000 pairs of birds living on the cliffs, representing more than 20 species. These include Atlantic puffins. Lots of sea life can be seen as well, including porpoises, dolphins, grey seals, whales, and sharks.

The Cliffs of Moher are a popular setting for filming, and have appeared in many popular movies including The Princess Bride and Harry Potter. They have also been featured in many music videos.

16 - The "Cotton Castle": Pamukkale

Pamukkale, meaning the "cotton castle" in Turkish, is located in Denizili in southwestern Turkey. It is a natural hot springs that are made of travertine, a sedimentary rock. It is most famous for the carbonate mineral that the flowing water leaves behind.

Pamukkale contains 17 hot springs in which the temperature ranges from 35-100 degrees Celcius. The spring water is transported 320 meters to the head of the terraces, depositing calcium carbonate on the first 60-70 meters. The calcium carbonate is deposited by the water as a soft gel, and it eventually crystallizes into travertine.

Legends say that this natural wonder was created by cotton (the area's principal crop) that was left out to dry by giants. The ancient Greco-Roman city of Hierapolis was built on the top of this "castle". It can be seen from the hills on the opposite side of the valley.

Tourism is a major industry in the area. At one point, hotels were built over the ruins if Hierapolis, causing lots of damage. A road was also built over the slopes. Both were demolished and removed when Pamukkale became a World Heritage Site in 1988.

15 - Active and Simmering: Mount Nyiragongo

Located inside Virunga National Park in the Democratic Republic of Congo, Africa, Mount Nyiragongo is an active stratovolcano with an elevation of approximately 3500 meters. A stratovolcano is a cone volcano that is built up by many layers of lava, pumice, tephra, and ash. The lava lake that it contains is the most voluminous in history, recorded at 600 meters prior to eruption in 1977.

The lava that is emitted from the volcano is unusually fluid and may reach speeds of 100 kilometers per hour when flowing down the volcano's sides. The low silica content of the lava causes it to be less viscous than lava from most volcanos.

Not much is known about how long Mount Nyiragongo has been erupting, but since 1882, 34 eruptions have been recorded. Churning lava lakes in the volcano's center often occur as continuous activity. The last eruption occurred in 2002.

The volcano is constantly monitored by a team of scientists at the Goma Volcanic Observatory. It is monitored 24/7. Seismic data is produced every four minutes and temperature data is produced every ten minutes.

Despite the fact that this volcano is one of the most dangerous in the world, the area at the foot of Mount Nyiragongo is dotted with highly populated villages, including the village of Goma.