Step 1: Describe it from a psychological angle:
The third level can be understood as Charley's psychological escape route from the stress and insecurity of contemporary life, imagined as a physical place within the station.
Step 2: Final Answer:
It was a supposed hidden level of Grand Central Station that opened into the past (1894), situated, according to Charley, somewhere beyond the station's known first and second levels, symbolising a longing to escape present-day anxieties.