Great unknowns Huge questions remain unanswered about life on Earth. We asked leading scientists and conservationists: what is the one thing you would like to know about the planet that remains a mystery?

"How many species of animals do we share the planet with? Estimates range from 3 million to as many as 100 million, and there's not much sign that we are converging on an answer."

- Patrick Greenfield

How many species are there on Earth?

Prof Andy Purvis is a researcher at the Natural History Museum

I'd go back 540m years to see the biological big bang'

"As an evolutionary biologist, I would love a time machine to go back to the Cambrian explosion [when most major animal groups first appeared in the fossil record] to see why this short period resulted in the really rapid rise of most animal groups, and why some like trilobites [extinct marine arthropods] didn't survive." Evolutionary biologist Dr Corrie Moreau is an expert on ants at Cornell University's Moreau lab 

Could some of the smallest life forms help avert climate crisis?

"Just as we humans rely on gut microbiomes for good digestive health, the dirt beneath our feet contains an uncountable number of bacteria, fungi and viruses that influence the health of soil and the plants that grow in it. Because most of these organisms can't be cultured in the lab, we know very little about their ecology. Yet the presence of particular microbes can help trees grow up to three times faster. Could these 'good microbes' be allies in fighting climate change and promoting food security?" Dr Bonnie Waring is a senior lecturer at Imperial College London's Grantham Institute

What is the full biodiversity of the Amazon or Congo basin rainforests?