Vietnam’s Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle. Despite its size, Son Doong wasn’t discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009. (Photo by Jason Speth/HuffPost)

A cyclist rides in al-Qudra desert in the Gulf emirate of Dubai, on June 8, 2020. (Photo by Giuseppe Cacace/AFP Photo)

The DFDS King of seaways enters the Mouth of the Tyne on the North East coast in Rough sea on Wednesday, January 18, 2023. (Photo by Owen Humphreys/PA Images via Getty Images)

A photograph of Cuba's former President Fidel Castro decorates a wall inside a state-run market in Havana, March 8, 2016. (Photo by Alexandre Meneghini/Reuters)

Mary Hickey poses with her collection of dolls at her home in Ashbourne, Ireland October 9, 2013. Hickey has been collecting the dolls for over thirty years and now owns over 420 dolls. (Photo by Cathal McNaughton/Reuters)

This handout photograph taken and released by Limex Images for Red Bull on February 6, 2019 shows France's Nicolas Ivanoff flying over the skyline prior to the first stage of the Red Bull Air Race World Championship in Abu Dhabi, United Arab Emirates. (Photo by Predrag Vuckovic/AFP Photo)

Five-month-old baby elephant Fah Jam swims during a hydrotherapy treatment as part of a lengthy rehabilitation process to heal her injured front left foot at a rehabilitation center in Pattaya, Thailand January 5, 2017. The baby elephant was injured at three months old when she got stuck in an animal snare put up by villagers to prevent elephant intrusions in Chanthaburi province. (Photo by Athit Perawongmetha/Reuters)

A migratory bird picks her food as army soldiers are reflected in a fountain during rehearsals for the upcoming Republic Day parade at the Raisina hills, the government seat of power, in New Delhi, India, Monday, January 18, 2021. India celebrates Republic Day on Jan. 26, highlighted by a march past by different branches of the military as well as a display of arms and missiles. (Photo by Manish Swarup/AP Photo)