Bellman's lost-in-a-forest problem
Bellman's lost-in-a-forest problem is an unsolved minimization problem in geometry, originating in 1955 by the American applied mathematician Richard E. Bellman. The problem is often stated as follows: "A hiker is lost in a forest whose shape and dimensions are precisely known to him.
Source: Wikipedia — Bellman's lost-in-a-forest problem (CC BY-SA 4.0)