Nothing is completely flat. Not even time. There are tiny wrinkles in time. Down to the smallest of scales, even smaller than atoms, there is quantum foam. The home of the wormholes.
Sadly, these wormholes are much too small for a human to pass through, but this is where we need time machines. Some say that we could capture a wormhole and enlarge it many trillions of times so that a human or spaceship to pass through it.