How did he know?

[theadimail]

No: HPFGUIDX 146797

I was wondering about how Voldemort could have found about the spell 
to make horcruxes eventhough it is supposed to be such a scret and 
unmentionable subject. He found it even while he was at Hogwarts, so 
how could a teen, no matter how talented, find such a thing without 
arousing the interest of Dumbledore who was, as he says, has been 
watching Tom Riddle, all the time,during his school days?

Bye
Adi