Three events/mysteries come to mind:
The Taos Hum - Taos is a small town in New Mexico, USA. Since the 1990s, the residents have reported hearing a continuous low frequency sound. It’s a rumbling sound comparable to the sound of a distant diesel engine. Although the naked ear can hear it, sound detection devices have not been able to pick it up. No one has been able to give an explanation about the origin of the sound. By the way, Taos is also the hometown of actor Julia Roberts.
The ‘Wow’ signal - In 1977, Jerry Ehman, who was a volunteer for SETI, the Search for Extraterrestrial Intelligence, recorded a ‘Wow’ signal, which lasted for 72 seconds. It is believed to have originated within the Sagittarius constellation near a star called Tau Sagittarii, 120 light years away. The signal was not noise and is the strongest proof of extraterrestrial life ever found. However, all attempts to locate the signal again have failed.
The Pollock sisters - In 1957, two young sisters, Joanna Pollock and Jacqueline Pollock, aged 11 and 6 respectively were killed in a car accident in Hexham, England. Their mother gave birth to twins, Gillian and Jennifer a year later. When the twins began to talk, they started asking for toys that Joanna and Jacqueline had played with. They could identify places Joanna and Jacqueline had been to. And they were scared of cars. More mysteriously, Gillian and Jennifer had birthmarks on their bodies exactly where Joanna and Jacqueline had them.