Episode 576

"Z's Ambition Arc - The Mysterious Strongest Army Corps Appears!" is the 576th episode of the One Piece anime.

Short Summary
The marines that had surrounded the Straw Hats were not really there, it was a mirage however nami warned them that they cannot let their guard down just because it was a mirage. They were in the Maubeugemour sea also known as the "ill tempered sea" home to weather conditions you've never seen or heard before.