This is what I believe to be true, yet it should be noted that I have a rather vivid imagination:

furthermore ipscanning is illegal in some places. Underground sites would be mainly oddport,
Forcing you to portscan (also illegal in most places)
ssl, and password protected ...

not to mention the amount of ip's out there ... 60% of (2^32)
Then were not talking about the hidden services of the onion router project
Sites ran on LAN's accessible only through VPN
hamachi networks ..
If i was desperate to get into "underground" sites i would try the front door
or at least limit myself to port scanning(yes it would still illegal) domains in the no-ip group, or similar ones
Because as far as i know they don't rely on obscurity, just on plain oldfashioned cryptography

basicly what you are trying to do is
(Bara Stegano)^32

thank you, guys. i didnt know about those things. i should read some tutorials. i still must learn how internet is built