Groundwater pollution has become a serious environmental problem.

I saw various aquatic creatures at the aquarium.

He is exactly in the state of 'A frog in a well does not know the vast ocean.'

He spoke of his big dreams, saying, 'How can a sparrow understand the ambitions of a swan?'

He crossed many mountains and rivers to meet her.

Please remember this old proverb, 'The friendship of a gentleman is insipid as water.'