The Doctor travels to the end of the universe to learn what causes fear, following Clara's telepathic trace. But it's Clara who learns what the Doctor is truly afraid of.
The Philadelphia Experiment -- UFO investigator Morris Jessup received a letter in 1956 claiming the Navy conducted a test in teleportation and invisibility during WWII. The experiment supposedly made the USS Eldridge temporarily disappear.
Ape-Man Army -- Early in the 20th century, Soviet scientist Dr. Ilya Ivanov experimented in cross breeding humans with apes. His goal was an army of soldiers as smart as humans, but as strong as gorillas.
Zapped to Death -- Thomas Edison's efforts to gain the upper hand in the electrification of America including gruesome publicity stunts like electrocuting an elephant to death and developing the electric chair.