unsolved

It is one of the world's most famous unsolved codes whose answer could sell for a fortune -- but two US friends say they have already found the secret hidden by "Kryptos."

Plus, there are still unsolved energy constraints facing the AI boom.

The killings in the Valley of the Christians are also unsolved.

This means high-risk, massive capital requirements, unclear timelines and unsolved fundamental technical problems.

Predicting superconductivity -- especially in materials that could operate at higher temperatures -- has remained an unsolved challenge.