What if [the Ghost] tempt you toward the flood, my lord,
Or to the dreadful summit of the cliff
That beetles o'er his base into the sea,
And there assume some other horrible form,
Which might deprive your sovereignty of reason
And draw you into madness? Think of it.
The very place puts toys of desperation,
Without more motive, into every brain
That looks so many fathoms to the sea
And hears it roar beneath
Here, in Act 1, Scene 4 as Hamlet contemplates following the Ghost that has appeared, Horatio prophetically describes castle at Elsinore as a potential source of suicide risk, as is discussed further in Quotes by Setting: Elsinore, Denmark (the third quote).
Something is rotten in the state of Denmark.
Here in Act 1, Scene 4, in one of the most famous lines from Hamlet, the watchman Marcellus describes the underlying theme that all is not right in Denmark in terms of smell. You can read more about this quote in Famous Quotes Explained, in Quotes by Setting: Elsinore, Denmark (the fourth quote) and in “The Ill Health of the State,” which is the sixth theme of the play.