A horse named Quinn, with a skeleton painted on it to help educate officers, is pictured during a visit by Britain's Queen Elizabeth II (unseen) and Britain's Prince Charles, Prince of Wales (unseen) to the Household Cavalry Mounted Regiment at the Hyde Park Barracks in west London on October 24, 2017. (Photo by Chris Jackson/AFP Photo)

The Rio de Janeiro Olympics kick off on August 5 for some two weeks of competitive games drawing athletes as well as audiences from around the world. As they prepare to cover the event, Reuters photographers look back on their favourite pictures from past Olympics where they captured the action on the ground as well as the participants' tears and joy that followed. (Photo by Lucy Nicholson/Reuters)

Undated David Yarrow handout photo of a gorilla as the self-taught wildlife photographer promotes his book, Encounter. (Photo by David Yarrow/Clearview/PA Wire)

“The scientists behind NASA's $2.5 billion Curiosity rover mission on Mars on Tuesday explained the nature of a tiny, gleaming "flower" embedded in Red Planet rock, and revealed where they'll be using the SUV-sized robot's drill for the first time”. – Alan Boyle via NBCNews.com

Photo: Scientists say that a “Martian flower”, seen here in an image from the Curiosity rover's Mars Hand Lens Imager, is a 2-millimeter-wide grain or pebble that's embedded in the surrounding rock. Another, darker-colored mineral grain can be seen above and to the left. (Photo by NASA)

A large bull walrus returns to the shores of Prins Karl Forland after diving and feeding on clams. (Photo by Paul Nicklen/National Geographic). Svalbard, Norway, 2011

An incredible view of daytime lightning on Saturn during one of the biggest storms the ringed planet has ever seen has been captured by a NASA spacecraft.

Photo: The huge storm which generated the lightning strike on Saturn can be seen in the top right of this image. (Photo by NASA)

These incredible pictures look like a bird's-eye view of a faraway alien planet – but they're actually hot springs. The amazing snaps are up-close shots of Iceland's hot springs, and the point-blank pictures produce a breathtaking array of colors and effects. Here: The amazing snaps are up-close shots of Icelandic hot springs. (Photo by Johann Vilhjalmsson/Caters News)

“After an afternoon of hiking and exploring the caves of Rhossili Bay, West Glamorgan, we caught the sun breaking through the clouds”. (Photo by Megan Warren-Davis/The Guardian)