In differential geometry, a Nadirashvili surface is an immersed complete bounded minimal surface in R3 with negative curvature. The first example of such a surface was constructed by Nikolai Nadirashvili in Nadirashvili (1996). This simultaneously answered a question of Hadamard about whether there was an immersed complete bounded surface in R3 with negative curvature, and a question of Eugenio Calabi and Shing-Tung Yau about whether there was an immersed complete bounded minimal surface in R3.
Hilbert (1901) showed that a complete immersed surface in R3 cannot have constant negative curvature, and Efimov (1963) show that the curvature cannot be bounded above by a negative constant. So Nadirashvili's surface necessarily has points where the curvature is arbitrarily close to 0.