Isn't enlightenment the ultimate death?

Is it true that after you reach enlightenment you simply vanish into the vast space of existence, and you simply cease to exist? Kind of like, when you have not attained to enlightenment you keep on returning and being reincarnated, but if you reach enlightenment and stop being reborn don't you just cease your existence and die forever? If I remember correct the Buddha did not know what happens to an enlightened person after his death, or am I wrong?