system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The cost of the proposed NYC 2nd avenue subway line is $17 billion, or about $2 billion/mile. This is around five times as expensive as similar completed urban subway projects around the world even before inevitable cost overruns that have already occurred. What's the deal?
gpt
There's a few main reasons:

* The age of NYC (it's been settled for ~400 years) and the tunnels mean that you run into all kinds of tunnels, utility pipes, and other objects that you don't know are there. This, combined with the fact that you have one of the most densely populated cities in the world above it makes construction both difficult and dangerous.

* The reason NYC can have so many tall buildings is that the island of Manhattan is essentially one huge slab of granite bedrock, as opposed to most cities on the water, which have relatively mud-like soil. This is very slow, difficult, and expensive to tunnel through, though it makes for better tunnels in the long run.

* Because of the danger associated with this work the labor unions have demanded high wages to offset the potential risk to workers, making for very high construction costs.

* New York City is one of the most expensive cities in the world to live in. Everything, from a cup of coffee to building a subway tunnel, is going to be significantly more expensive then just about anywhere else.

* Big cities, and New York City in general, are known for a ton of red tape being involved in construction projects, which increases the costs significantly.