Earth's deepest rocks help define upper limit for viscosity beyond which materials effectively become rigid

Viscosity is one of the most fundamental physical properties used to describe how materials flow.

It governs the movement of liquids, molten rocks and even slowly deforming regions deep inside the Earth.

While scientists have long studied materials with low or moderate viscosities, a simple but important question has remained largely unexplored: Is there a physically meaningful upper limit to viscosity? ...