From The TimeSplitters Wiki

Kronia is the name of the city and base located under the Scottish island in 1924. It is also in this place that the Time Crystals were found and used by Crow. It holds many rooms, including a Time Machine Chamber, A docking bay (for the submarine), an entire underground city and a warehouse, where many robot prototypes (Robot Louis Stevenson) are held.

It is unknown if Kronia existed before Crow found it, although because of the name, it would seem that he found it. It also unknown how he knew that the Time Crystals were under the island, and it is also unknown how Crow managed to travel back to 1924 to mine the Crystals in the first place, which creates some paradoxes.