Sink, Sank, Sunk
The highlighted verb is intended to be in the simple present tense, so it should be sank (Straits Times online, 12 August 2011).
Sunk is, of course, the –en/ed participle of the verb sink, which has irregular past (sank) and –en/ed participle (sunk) forms, so we do not say His heart *sinked/*has sinked.
The –en/ed participle is more commonly known, especially in schools, as the past participle, but this is inaccurate since past tense is not inherent in the verb form, as the following examples illustrate:
(1) The yacht has sunk. (present perfective)
(2) The yacht had sunk. (past perfective)
(3) The yacht will have sunk. (modal perfective, referring to future time)
Note that, in all three cases, the verb sunk remains unchanged and any tense marking is left to the other, auxiliary verbs.