Today's Geo Quiz is about as big as it gets. We're looking for an area that makes up 10 percent of the Earth's land surface.
This vast region spans much of northern Asia and Russia. This landmass covers about 75 percent of Russia's territory. Yet it's home to 40 million people, or only about a third of Russia's population.
The Ural Mountains stand tall on its western border with vast stretches of frozen tundra extending all the way to the Pacific in the east. This wide open territory boasts the world's deepest freshwater lake Lake Baikal, a trans-continental railway, and it contains huge deposits of minerals and natural gas.
Lake BaikalLake Baikal
It's also home to thousands of species of migratory birds -- including one called a Chiffchaff that we'll tell you more about here...
Most people who love birds are content to occasionally fill their bird feeders and see whatever comes along.
Siberian Chiffchaff - photo courtesy London Wildlife TrustSiberian Chiffchaff - photo courtesy London Wildlife Trust
Listen to the Chiffchaff bird song (.ogg file)
Other bird watchers devote hours of each day to keeping an eye out for birds, in the hope of a rare sighting.
Mark Pearson is in the second category. He's the community officer for London's Wildlife Trust, a charity that aims to protect London's wildlife and wildspaces for the future. Pearson was out walking the other day (in the East Reservoir Community Garden in Stoke Newington) when he spotted an unusual bird -- a Siberian Chiffchaff -- more than 3000 miles off its usual migratory path, or from its home habitat in Siberia, the answer to our Geo Quiz.
The Siberian Chiffchaff is one of the rarest species spotted in this area in a long time says Pearson, along with an Iceland Gull which was sighted last month. To confirm its identity, Pearson studied its plumage, took some good quality photos, and then managed to elicit the bird's contact call using his mobile phone. Have a listen to his story!