The history of man's exploration of the earth extends over 5,000 years.The earliest cavemen explored in a very limited way:they had to go out to gather plants and hunt animals to feed themselves.(1)_______________________.But the wider world remained a mystery to them.It was only later,once the necessities of life had been obtained,that people began to wonder what lay on the other side of the hill,whether the forest around them ever came to an end,or where the sun went after it had fallen into the sea.This curiosity caused them to discover previously unknown lands and seas.(2)__________________________________,but this does not mean there is nothing left to explore on Earth.It is true that with the invention this century of jet aircraft and spaceships,every part of the world has been photographed and maps made of it.But there is still much left to find out.Enormous parts of other continents remain practically unknown.And yet all these places are nowadays comparatively easy to reach.(3)_______________________.The universe is as mysterious and exciting for them as our planet seemed to the people in the past.Man's natural curiosity drives him on to explore the unknown and to travel where no one has traveled before, even at the risk of his life.
