For centuries rumours have persisted about a powerful and mysterious substance. And these days, adverts and videos offering it for sale can be found online. Why has the story of "red mercury" endured?
Some people believe it's a magical healing elixir found buried in the mouths of ancient Egyptian mummies.
Or could it be a powerful nuclear material that might bring about the apocalypse?
Videos on YouTube extol its vampire-like properties. Others claim it can be found in vintage sewing machines or in the nests of bats.
There's one small problem with these tales - the substance doesn't actually exist. Red mercury is a red herring.