Introduction
The narrator in the story "The Third Level" describes Grand Central Station as a growing tree. He makes this comparison because of the station's complex network of tunnels and passages that resemble roots and branches.

Reasons for the Comparison
The narrator compares Grand Central Station to a growing tree because of the following reasons:

1. Size: Just like a tree grows and expands its branches, Grand Central Station has grown in size over the years. The station has undergone many renovations and expansions to accommodate the increasing number of passengers.

2. Network of Tunnels and Passages: The station has a complex network of tunnels and passages that connect different parts of the station. These tunnels and passages resemble the roots and branches of a tree.

3. Timelessness: The narrator talks about how Grand Central Station has been around for more than 40 years, and it has stood the test of time. Trees also have a timeless quality and can live for hundreds of years.

Why has it been an exit?
Grand Central Station has been an exit for many reasons:

1. Transportation: Grand Central Station serves as a major transportation hub in New York City. It connects to various subway lines, buses, and trains, making it easy for people to travel to different parts of the city.

2. Location: The station is located in the heart of Manhattan, making it easily accessible to millions of people who live and work in the area.

3. Landmarks: Grand Central Station is also a popular tourist destination. It is home to many landmarks, including the iconic clock in the main concourse and the whispering gallery.

Conclusion
In conclusion, the narrator compares Grand Central Station to a growing tree because of its size, network of tunnels and passages, and timelessness. The station has been an exit for many reasons, including its importance as a transportation hub, its location in the heart of Manhattan, and its landmarks that attract tourists from around the world.