conundrum
n
1 question, usu with a pun in its answer, that is asked for fun; riddle 问题(答案通常为双关语的); 谜语.
2 puzzling problem 伤脑筋的问题
an issue that is a real conundrum for the experts 使专家们大伤脑筋的问题.
Shortly after this the famous H. M. S. Challenger expedition established the study of the sea-floor as a subject worthy of the most qualified physicists and geologists.