Great idea for a thread.
Wow. Number stations, just had a listen to Lincolnshire Poacher and Cherry Ripe. I also read how the code is uncoded (Wiki explanation) but my brain started going la la la due to it going waaay over my head. Obviously not cut out for the intelligence business.
Big cats (where I live there are rolling moors nearby). Yes - anything to do with birthing is just amazing.
Terrorism (by which I mean psychology of how people get radicalised, not that I want to throw a bomb at anyone).
WW1 & 2 - understanding how ordinary people experienced these, and great social upheavals like partition (India).
Boris - brilliant podcast on BBC currently about his life from being a child.
Victorian death portraits. Wisconsin Death Trip is a dark, dark book of fascination. I have to keep it in the attic away from more innocent books. I mean I would have a hard time seeing it next to little house on the prairie.
Missing people - those who walk out of their lives and are never found.
The Russian mystery of the group who went camping and possibly attacked by a bigfoot (well, that is one theory, 2019 investigation concluded it was an avalanche) at Dyatlov Pass.