SHODAN (http://www.shodanhq.com/) is a search engine that allows find specific computers (routers, servers, etc.) using a variety of filters. They grad this "horrible" data from (routers, servers, etc.) 'banners'.
Using this DB, hackers can find SCADA Internet-facing Web interfaces, default passwords for web servers and network devices, IP cameras, vulnerable systems (filtering by IIS 5, windows 200, etc), and many more.
Some interesting SCADA information (tooked from SHODAN DB):
Using this information, H4ck3rs can locate these critical national infrastructures systems and try to penetrate them, what can be sometimes very easy.
Here is one nice example:
Here are some default password in use:
Please don't use this data to hack these systems, this is illegal !!