The Darkside
The cold and airless Darkside of Mercury is little known, as only a few travellers have entered that region and survived. Currents generated by Mercury's passage through the Sun's electromagnetic field prevent radio communication and would damage the instruments of any rocket that attempted to land there. Furthermore, for some time, those who returned were afflicted by a morbid psychosis that was fancifully attributed to "demons". This was eventually traced to a form of intelligent life inhabiting a crystalline matrix and possessing the power to affect mind-waves. These life-forms have since become quiescent and no longer pose a hazard (The Demons of Darkside).
Read more about this topic: Mercury In The Fiction Of Leigh Brackett