We have studied the spin correlations with k= ( 1 2 , 1 2 , 1 2 ) propagation vector which appear below 0.4 K in Tb2Ti2O7 spin liquid by combining powder neutron diffraction and specific heat on Tb2+xTi2−xO7+y samples with x=0, 0.01, -0.01. The k= ( 1 2 , 1 2 , 1 2 ) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1) K. Refining the subtracted patterns at 0.07 K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 µB ordered on a length scale of about 20Å. In addition, long range order involving a small spin component of 0.1 to 0.2 µB is detected for the x= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ( 1 2 , 1 2 , 1 2 ) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.Rare earth pyrochlores with geometrical magnetic frustration are model compounds to investigate classical spin ice as well as quantum spin ice physics. The short range ordered ground states with large degeneracy give rise to a large variety of interesting phenomena such as magnetic monopoles 1,2 , first order transition 3 , or unconventional magnetoelastic excitations 4,5 . These frustrated ground states are highly sensitive to small perturbations and to the presence of defects. Already in the 80s, Villain 6 predicted that for an arbitrary small concentration of defects the spinel lattice with pyrochlore structure could switch its ground state from spin liquid to spin glass. More recently many studies focus on the role of a minute amount of defects in pyrochlores. The crystal structure of R 2 Ti 2 O 7 pyrochlores (where R 3+ is a rare earth ion, Ti 4+ is non magnetic) accommodates oxygen vacancies, small off-stoechiometries in the R 3+ or Ti 4+ lattice, "stuffing" or site inversion. Oxygen vacancies introduce Ti 3+ defects instead of Ti 4+ ions, modifying the R 3+ crystal field and inducing local distortions, with various consequences on the low temperature magnetic properties and spin ice physics. In classical spin ice Dy 2 Ti 2 O 7 , oxygen depletion or a low level Dy stuffing in the Ti sites strongly slows down the monopole dynamics at very low temperature 7,8 and affects the nonequilibrium macroscopic properties 9,10 . In the quantum spin ice Yb 2 Ti 2 O 7 , small off-stoechiometry clearly affects the magnetic ground state 11 . It appears that precise studies of the magnetic defects are needed to resolve controversies concerning these unconventional ground states.Among the pyrochlores, Tb 2 Ti 2 O 7 is one of the most complex, the subject of numerous investigations since the discovery of its spin liquid character in 1999. The Tb 3+ short range correlated moments fluctuate 12 down to 20 mK at the time scale of the muon probe (10 −6 s). The origin of these fluctuations has been highly debated [13][14][15][16][17][18] . Several mechanisms have been pr...