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Nearly blind, they rely on echolocation - high-pitched sound pulses that bounce off objects and return as echoes - to navigate murky waters.

But there's another sound cutting through the trees of the Zambian bush - the unmistakable high-pitched whine of a bitcoin mine.

The voice could grumble and soar, scraping the deepest recesses of evil and reaching the high-pitched cajoling of a championship schemer.

The high-pitched electronic beep is usually only heard during foggy conditions, most clearly at Land's End, and also by residents of places around the coast including Sennen and Cape Cornwall.

“Why do people think that sweet is round … that bitter is angular and sour is high-pitched?”