Understanding the Question:
This refers to Jack Finney's story 'The Third Level', about Charley, who discovers (or imagines) an extra level at Grand Central Station.
Final Answer:
The 'third level' refers to a mysterious, extra level of the Grand Central Station that the narrator, Charley, claims to have stumbled upon — one that seems to belong to the year 1894, complete with old-fashioned clothes, gas lamps, and a newspaper dated to that year. Officially, Grand Central has only two levels, so the third level is understood as a psychological escape, a product of Charley's (and modern man's) subconscious longing to flee the anxiety, insecurity and pressures of the atomic age into a simpler, more peaceful past.