Synonyms for «old mystery»

Synonyms for individual words can be found below.

In fact, I have cracked the age - old mystery of why resumes are just so dreadful. (ngcareerstrategy.com)
The case is finally closed on a 25 year old mystery. (vgtribune.com)
The aim of the exercise is to solve a decades - old mystery about the part that friction plays in such an event. (scientificamerican.com)
(see all sentences)