Final Fantasy XV includes more areas than those you are allowed to visit and explore in the game. Fortunately, today's gamers usually finds a way around this, and that is exactly what the Youtuber Fotm Hero has done.
Thanks to a glitch, he/she has entered vast areas of Niflheim that were not part of the actual adventure. The video is roughly an hour, but to save you time, there are timestamps included in the description of the juicy stuff:
0:00 : Cartanica
12:33 : 3 kilometers before Cartanica station
16:34 : 5 kilometers before Cartanica station (African Savannah looking area.)
24:32 : Eusciello (snowy area.)
39:30 : Area where Prompto falls from the train.
50:56 : Pagla, area where Noct is having a conversation with Ignis on the phone while being on top of the train.
56:05 : Tenebrae.
We assume at least some of them (or even parts of these parts) will be used in upcoming DLC, one way or another. Still they may also remain unused for one reason or another.