Groundwater pollution has become a serious environmental problem.

I saw various aquatic creatures at the aquarium.

He took action to stir up unnecessary problems, like poking a bush and bringing out a snake.

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

He crossed many mountains and rivers to meet her.

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