• Lists of unsolved problems (16 P)
• Year of death unknown (1 C, 9,305 P)

B

• Bridge disasters of unknown cause (4 P)

C

• Undeciphered historical codes and ciphers (1 C, 20 P)
• Unsolved crimes (7 C, 5 P)

D

• Date of birth unknown (2,229 P)
• Date of death unknown (1,339 P)
• Unsolved deaths (29 C, 95 P)
• Unexplained disappearances (8 C, 18 P)

E

• Economic puzzles (19 P)

H

• Hypotheses (14 C, 96 P)

I

• Inscriptions of disputed origin (2 C, 22 P)

L

• Unsolved problems in linguistics (6 C, 10 P)
• Lost inventions (1 C, 16 P)
• Lost objects (7 C, 20 P)
• Lost places (5 C, 5 P)

P

• Unidentified people (11 C, 138 P)
• Philosophical problems (5 C, 45 P)
• Place of birth unknown (885 P)
• Place of death unknown (507 P)

S

• Scientific problems (8 C, 4 P)
• Year of song unknown (770 P)

T

• Unknown-strength tropical cyclones (4 P)

U

• U-boats sunk by unknown causes (31 P)
• Unexplained phenomena (8 C, 35 P)
• Unsolved problems in mathematics (6 C, 76 P)

W

• Works of unknown authorship (10 C, 266 P)

Y

• Year of birth unknown (1 C, 26,035 P)
• Year of establishment unknown (42 P)
• Year of introduction unknown (6 P)

• Open problem

*

• Lists of problems

A

• Almáttki áss
• Alpha effect

B

• Binding problem

C

• Certificate revocation
• Controversies in autism

G

• Great Filter

H

• Hard problem of consciousness

O

• Overlapping markup

P

• Philosophy of artificial intelligence
• Potential applications of carbon nanotubes
• Problem of mental causation
• Publius Enigma

Q

• Quantum computing
• Glossary of quantum computing

S

• Space colonization

T

• Terraforming

W

• Why am I me