It's well known that the combinatorial explosion means that there are many, many, possible chess games. And yet most games are decided within 100 moves, and Wikipedia says that the longest tournament chess game took 269 moves. And most positions can be reached relatively quickly.
So what's a legal chess position that takes the most moves to reach? It may be extremely difficult to prove that it takes the number of moves you say; please provide either a proof game or a description explaining why it takes as many moves as you say.
For the purposes of this puzzle, the 75-move rule does not apply.