these mysterious prime numbers

i chanced upon a few prime numbers  while checking for
 prime in the link on the top right of this blog. astonishingly
the first no. i tried was prime. i got 6 more prime numbers in as
many minutes. strange isnt it ? have a look at the numbers
which are all prime. see the build up . astonishing !

173573
1735733
17357339
173573333
17357333377
1735733333377
17357333377337