WHOIS information is based upon a tree hierarchy. ICANN (IANA) is the authoritative registry for all of the TLDs and is a great starting point for all manual WHOIS queries.
* [Search Engines for Academic Research](https://www.itseducation.asia/deep-web.htm)
* See additional information under the [OSINT Dark Web OSINT Tools section](https://github.com/The-Art-of-Hacking/h4cker/tree/master/osint#dark-web-osint-tools)
### Other Great Intelligence Gathering Sources and Tools
* Resources from Pentest-standard.org - http://www.pentest-standard.org/index.php/PTES_Technical_Guidelines#Intelligence_Gathering
## Active Recon
* Tons of references to scanners and vulnerability management software for active reconnaissance - http://www.pentest-standard.org/index.php/PTES_Technical_Guidelines#Vulnerability_Analysis