Over 15,000 photographers entered this year's National Geographic Travel Photographer of the Year Contest, which showcases some of the best images captured over the past two years in locations around the world. Judges pared those submissions down to a short list of 18 photos, broken into three categories: nature, cities and people.

Grand prize and 1st prize, nature category: The Power of Nature, Sergio Tapiro

Grand-prize-winner Sergio Tapiro was in the town of Comala, Mexico, in December 2015 when the Colima volcano began to blow. A powerful explosion spewed out a cloud of ash at the same time as a massive lightning bolt illuminated the dark scene. Tapiro told National Geographic it was "one of the most exciting moments of my life."

Jody MacDonald, one of three professional photographers chosen to pore over the thousands of submissions, said the photo combines exactly what the judges were looking for: a combination of technical skill, creativity, composition and originality.

"It was a very visually incredible photo and all of us being photographers, we understood the amount of time and effort that would've gone in to making that photograph," she said.

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(Sergio Tapiro/National Geographic Photographer of the Year)

1st prize, cities category: Levels of reading, Norbert Fritz

Hungarian photographer Norbert Fritz snapped this photo of the modern interior of the city library in Stuttgart. Fritz captures the library's wide open, airy central atrium, where natural light floods in through the windows on the roof. 

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(Norbert Fritz/National Geographic Travel Photographer of the Year)

2nd prize, cities category: Walled City #08, Andy Yeung

Yeung earned second prize in the cities category for this aerial view of Whampoa Garden, Hong Kong. Yeung told National Geographic he drew inspiration for the photo from Hong Kong's former Kowloon Walled City, once one of the densest places on Earth until it was finally demolished in 1990.

"If you look hard enough, you will notice that the city is not dead. Part of it still exists in many of current high density housing apartments," Yeung told the magazine.

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(Andy Yeung/National Geographic Photographer of the Year)

3rd prize, cities category: Henningsvær Football Field, Misha De-Stroyev

​De-Stroyev earned third prize for his photograph of a soccer field in Henningsvær in Norway's Lofoten Islands, which is considered one of the most amazing pitches in Europe.

De-Stroyev snapped the photo during a 10-day sailing trip in Norway in June using a drone flying at about 120 metres above the ground, 

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(Misha De-Stroyev/National Geographic Photographer of the Year)

Honourable mention, cities category: Colourful Apartment, Tetsuya Hashimoto

Hashimoto took this photo of a vibrantly coloured apartment complex in the Gifu prefecture of Japan. It's one example of how the winning photographers transformed mundane spaces into engaging, original works of art, said MacDonald.

"Just being able to see the imagination and creative abilities of people and the efforts that people are going to to create compelling imagery is always very impressive," MacDonald said.

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(Tetsuya Hashimoto/National Geographic Photographer of the Year)

Honourable mention, cities category: Al Ain, Andrzej Bochenski

Bochenski took this picture in Al Burayami, Muhafazat al Buraymi, Oman. In it, a maze of new buildings appears to rise from the desert floor.

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(Andrzej Bochenski/National Geographic Photographer of the Year)

2nd prize, nature category: To live., Hiromi Kano

Kino's image of swans gliding over the water in Kabukurinuma, Osaki, Japan, nabbed the photographer second place in the nature category. Japan has lost many of its wetlands to real estate development and the Kabukurinama area has become a rare over-wintering place for birds.

Kino took pains to monitor the wind direction and fine-tune her camera's shutter speed to capture the strength and beauty of the birds' flapping wings — a shining example of the attention to detail shown by many of the "aspiring, very, very passionate photographers" MacDonald said entered the competition.

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(Hiromi Kano/National Geographic Photographer of the Year)

3rd prize, nature category: Crocodiles at Rio Tarcoles, Tarun Sinha

Sinha took his prize-winning image in Costa Rica while travelling from Monteverde to Playa Hermosa. While peering over the edge of a bridge, he saw 35 gigantic crocodiles relaxing on the muddy banks of the river.

"I wanted to capture the stark difference between the crocodiles on land and in the water. In the murky
waters, the body contours of these beasts remain hidden, and one can only truly see their girth as they emerge from the river," Sinha told National Geographic.

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(Tarun Sinha/National Geographic Photographer of the Year)

Honourable mention, nature category: Marble Caves, Clane Gessel

Gessel travelled to Patagonia with his father earlier this year. After a 10-hour drive on dirt roads, they finally came upon the sparsely populated Argentinian region's marble caves. Gessel chartered a boat to move his camera closer to the rock faces and waited for the light to be just right for capturing the caves' intricate blue swirls.

It's exactly the sort of image competition judge and National Geographic senior editor Molly Roberts said catches people's attention and draws them in.

"Often people are hoping that photographers will guide them to places that they otherwise wouldn't see," Roberts says. "That's what they love about it."

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(Clane Gessel/National Geographic Photographer of the Year)

Honourable mention, nature category: Forest of the Fairy, Yutaka Takafuji

Takafuji snapped this photograph at the end of a humid early summer day near a remote village in the Tamba area of Japan. She told National Geographic, "It beautifully captures the magical atmosphere of Princess fireflies carpeting a stairway leading to a small shrine revered by the local people."

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(Yutaka Takafuji/National Geographic Photographer of the Year)

Honourable mention, nature category: Mt. Bromo, Reynold Riksa Dewantara

The Mount Bromo volcano is a small, but active volcanic cinder cone on East Java, Indonesia. In January 2016, Dewantara was in Mount Bromo while there was an increase in seismic activity at the volcano and took this photo from the patio of a local hotel.

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(Reynold Riksa Dewantara/National Geographic Photogarpher of the Year)

Honourable mention, nature category: In Your Face, Shane Gross

Caribbean reef sharks are known to be camera-shy, so Gross placed his camera in a secluded area in the Gardens of the Queen, a marine area south of Cuba. He then used a remote trigger to click away as they came in — sometimes bumping against his camera.

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(Shane Gross/National Geographic Photographer of the Year)

1st prize, people category: worship, F. Dilek Uyar

Uyar took this photo of a whirling dervish performed a traditional dance called Sema in Konya, Turkey. The spinning represents the constant revolution of all living things.

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(F. Dilek Uyar/National Geographic Photographer of the Year)

2nd prize, people category: Interesting moment., Julius Y.

In this photo, visitors to the Rijksmuseum in Amsterdam mirror the curiosity present in Rembrandt's painting Syndics of the Drapers' Guild

It was surprising, candid moments like this that Roberts said she and the other judges were looking for.

"You're looking for something that's exceedingly well crafted and something that's visually strong, surprising. An unexpected moment," Roberts said.

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(Julius Y./National Geographic Photographer of the Year)

3rd prize, people category: Under the Wave, Rodney Bursiel

Bursiel recently travelled to the tiny South-Pacific island of Tavarua to take photos of professional surfer Donavon Frankenreiter. This photo was snapped at the Cloudbreak surfing break, just offshore from the tiny Fijian island.

"The usual surf shots have all been done so we decided to get a little creative," Bursiel told National Geographic. "Makes you look twice."

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(Rodney Bursiel/National Geographic Photographer of the Year)

Honourable mention, people category: Bridging Generation., Jobit George

In another example of mundane moments captured beautifully, a father and son share a moment together while dressed in white traditional attire on Eid al-Fitr at a mosque in New Delhi, India. 

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(Jobit George/National Geographic Photographer of the Year)

Honourable mention, people category: The Man's Stare, Moin Ahmed

Ahmed's photo was taken in July 2016 at Tongi Railway Station in Gazipur, Bangladesh. He noticed this pair of eyes peering through the window of a train from Dhaka, which had stopped for a few minutes during a downpour.

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(Moin Ahmad/National Geographic Photographer of the Year)

Honourable mention, people category: Blessings at Besakih, Michael Dean Morgan

Besakih Temple has been known as Bali's "Mother Temple" for over 1,000 years and is perched 1,000 metres high on the southwestern slopes of Mount Agung. Morgan took this shot of several Balinese people offering prayers and being blessed by the temple priests, or "Pemangku," who live there.

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(Michael Dean Morgan/National Geographic Photographer of the Year)