When the opposite is true
My friend and I went free-diving with 30-40 sharks in Hawaii with One Ocean Diving (highly recommend).
Sharks are apex predators, they have big teeth, and they are capable of killing humans.
It would be perfectly rational to imagine that the entire swim would be done with an elevated heartbeat, and fighting against fear of this reality.
When we pulled up in the boat, a number of the sharks came to the surface, and we got a first look at just how big some of them were (the larger, dominant sharks tend to swim closer to the surface). My heartbeat did start to quicken.
As soon as we got into the water through, looking down into the deep blue and the countless sharks below us, I felt my heart slow, and a deep sense of calming tranquility come over me. It was one of the most peaceful moments of my life.
Sometimes the opposite of our expectations is true, and this is often where magic lies.