The angel problem was posed in 1982, and little progress was made until it was solved independently and almost simultaneously in 2006 by four different people. To borrow from the Wikipedia article, the question is to determine the winner of a certain game:

The game is played by two players called the angel and the devil. It is played on an infinite chessboard. The angel has a power $k \in \mathbb{N}$ specified before the game starts. On each turn, the angel flies to a different square whose distance from its current square is at most $k$ in the infinity norm. The devil, on its turn, may permanently block any single square not containing the angel. The angel may fly over blocked squares, but cannot land on them. The devil wins if the angel is unable to move. The angel wins by surviving indefinitely.

The problem was first published in the book Winning Ways for Your Mathematical Plays by Berlekamp, Conway, and Guy and therefore became fairly well known among people interested in recreational mathematics. However, arguably the reason for the sudden appearance of four independent solutions in 2006 was the publication of Peter Winkler's book Mathematical Puzzles: A Connoisseur's Collection, which listed the angel problem as an unsolved problem, and presumably sparked the interest of a lot of people who had either not heard the problem before, or at least had not given it much thought.