Imaginary Number Space

Space (虚数空間) is an alternative space in the Nasuverse, apart from normal space and reality.

Principles
It is possible to travel through Imaginary Number Space in specialized craft such as the Imaginary Numbers Submersible Shadow Border, withthe use of the Imaginary Numbers Observation Device Paper Moon, though the method is difficult and inherently dangerous. In performing a 'Zero Sail' dive in this way, the craft departs from normal reality and dives into the 'sea' of imaginary numbers, and requires a connection or bond to be formed to an existence within the normal reality in order to surface.

Imaginary Number Space has 'tides' which can cause a time difference between vessels passing through it and normal reality.

Fate/Grand Order
Following the destruction of Chaldea at the hands of invaders from the Lostbelts, the survivors dove into Imaginary Number Space in the Shadow Border to escape the Oprichniki overwhelming the Antarctic coastline. After a week in Imaginary Number Space, they formed a connection to their attackers and managed to resurface in the Russian Lostbelt, with three months having passed for the planet.