A blue whale's heartbeat can be detected from over three kilometers away.
Why it is true
Due to the enormous size of its heart, the sound of the heartbeat is so powerful that scientists can hear it through the water using special hydrophones.
Source: Marine Biology
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.