Does bluetooth work underground?

yes, but its range will be much more limited

but as I assume you are just going from a phone or media player a few feet to the headset, it should make a difference.

the only way "underground" would affect you would be there was actual earth between your headset and the device
 
As long as the phone is with you on the subway & within 33 feet of of the headphones, they will work just exactly the same way they work above ground. You don't need a cellular nor WiFi signal in order for Bluetooth to work. You might not be able to make or receive calls, but the connection between the headphones & the cell phone will not be affected.
 
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