the is used with Sea, sky, ground, country

• The stars we see in the sky at night are unimaginably remote.

• It isn’t safe to go windsurfing when the sea is rough.

• My son fell off the swing at school and smacked his head on the ground.

• Would you prefer to live in a town or in the country?

but sea doesn’t take the when we mean travel or work.

• She was depressed the entire month that her husband was away at sea.

• We have been at sea for 10 hours.

Space doesn’t take the when we mean “the universe”, but it takes the when we mean “ empty place”

• John Glenn became the first person in history to eat a meal aboard a rocket in space.

• I noticed the space between his two front teeth when he grinned at me.

The + singular countable noun means a type, a group, a kind

• The elephant is the largest animal on land.

• The plane is the fastest means of transport.

• When was the telephone invented?