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.

Twenty-one people died and 220 were injured by bombs at the Mulberry Bush and Tavern pubs which remain unsolved.

He emphasizes that identifying the physical principles behind life's emergence from nonliving matter remains one of the greatest unsolved problems in biological physics.

Lib Dem Home Affairs spokesman Max Wilkinson said: "Every single day, thousands of victims are denied the justice they deserve... the government cannot afford to dither and delay, they must crack down on unsolved crimes."