From episode 239 "The Name of the Doctor": inside The Doctor's grave, we are shown a beam of light which is said to be "the scar tissue" of his journey through time. The Doctor describes the light as both his past and his future.
CLARA: What's that?
DOCTOR: What were you expecting, a body? Bodies are boring. I've had loads of them. Nah, that's not what my tomb is for.
CLARA: Doctor, explain. What is that?
DOCTOR: The tracks of my tears.
SIMEON: Less poetry, Doctor. Just tell them.
DOCTOR: Time travel is damage. It's like a tear in the fabric of reality. That is the scar tissue of my journey through the universe. My path through time and space from Gallifrey to Trenzalore.
*(He zaps it with his sonic screwdriver. Overlapping clips, sorry if I missed any.) *
DOCTOR 1 [OC]: Have you ever thought what it's like to be wanderers in the fourth dimension?
DOCTOR 4 [OC]: Do I have the right?
DOCTOR 6 [OC]: Daleks, Cybermen, they're still in the nursery compared to us.
DOCTOR 2 [OC]: There are corners of the universe that have bred the most dangerous things.
DOCTOR 9 [OC]: You were fantastic. Absolutely fantastic.
DOCTOR 10 [OC]: I'm the Doctor. I'm from Gallifrey in the constellation of Kasterborous
DOCTOR 11 [OC]: Hello, Stonehenge!
DOCTOR: My own personal time tunnel. All the days,
DOCTOR 3 [OC]: It was the daisiest daisy I'd ever seen.
DOCTOR: Even the ones that I, er, even the ones that I haven't lived yet.
Why is his grave his entire timeline from Hartnell to Smith, all of his adventures in his 13 faces and his future and not a body? Why can't the Doctor enter his timeline? What ones he haven't lived yet is he referring to? Why did the Great Intelligence think the best way to destroy the Doctor is his timeline?