Scanning for Google's internal corporate subdomains

For some reason Gmail appears to use an internal DNS server. This allows to verify existence and even resolve the IP addresses of Google's internal corporate domain names. For example, if you send an e-mail to test@root.corp.google.com you will receive an error response:

So apparently there's a server located at root.corp.google.com and its ip address is 172.16.115.10.
Here's a list of some of these domain names (of course it's not comprehensive).
Some companies:

yahoo.corp.google.com	172.31.9.1
oracle.corp.google.com	172.25.116.205
microsoft.corp.google.com	172.31.9.1
myspace.corp.google.com	172.25.121.235
bebo.corp.google.com	172.17.90.16
makani.corp.google.com	172.27.145.31
splunk.corp.google.com	172.25.117.23
netapp.corp.google.com	[::ffff:172.16.255.48], 172.16.255.48
tableau.corp.google.com	172.25.100.77
fireeye.corp.google.com	172.24.0.7

Google's corporate structure:

marketing.corp.google.com

172.18.77.12

And a lot of other words:

male.corp.google.com	172.25.208.116
offer.corp.google.com	172.25.121.53
girl.corp.google.com	172.22.73.34
computer.corp.google.com	172.26.77.190
fail.corp.google.com	172.25.121.235
fear.corp.google.com	172.17.80.52
death.corp.google.com	172.17.81.204
ash.corp.google.com	172.22.64.63
dust.corp.google.com	172.25.129.187
spirit.corp.google.com	172.17.90.106
policy.corp.google.com	172.24.184.119
nothing.corp.google.com	172.22.122.136
gypsy.corp.google.com	172.18.76.135
boot.corp.google.com	172.18.84.202
root.corp.google.com	172.16.115.10
surveys.corp.google.com	70.32.156.24
license.corp.google.com	172.24.98.14
humor.corp.google.com	172.22.115.121
peregrine.corp.google.com	172.18.116.84
rda.corp.google.com	172.17.90.30
sierra.corp.google.com	172.18.125.44
lattice.corp.google.com	172.22.116.27
manhattan.corp.google.com	172.18.135.203
research.corp.google.com	172.22.132.245
discovery.corp.google.com	172.22.96.46
concepts.corp.google.com	172.24.0.202
invent.corp.google.com	172.17.81.164
effort.corp.google.com	172.25.66.67
free.corp.google.com	172.18.104.61
kick.corp.google.com	172.22.133.156
air.corp.google.com	216.239.44.190
never.corp.google.com	172.22.102.87
event.corp.google.com	172.25.138.70
you.corp.google.com	172.17.132.179
aqualung.corp.google.com	172.22.118.62