The condensation occurs in the 370-390 o C range (Fig. ), coincident with the removal of organics (Fig. ). The structure of ZEO-3 was also solved ab initio by 3D ED with five datasets (Tab. S2-S3, Fig. ). The unit cell of ZEO-3 shrinks to a = 21.5046(8) Å, b = 21.2757(8) Å, c = 14.4638(4) Å, β = 108.7196(1)° but maintains the same symmetry as ZEO-2, while the topology of the chain is preserved. In fact, a 17% contraction of the structure occurs along a-and b-axis, while along c-axis only a marginal expansion (0.4%) occurs. To obtain more accurate atomic positions, the structures of ZEO-2 and ZEO-3, including the position of the disordered tCyMP in ZEO-2, were subsequently Rietveld refined against synchrotron powder X ray diffraction data (SPXRD, Figs. S4-S5, Tab. S4-S8). The final refined unit-cell compositions of ZEO-2 and ZEO-3 are [Si80O176H24]|(C19H36P)8 and Si80O160, respectively (see Supporting Material for details). The 1D pure silica chains in ZEO-2 are aligned along the [001] direction (Fig. ) and surrounded by four identical chains in the ab plane (Fig. ). At the edge of the ZEO-2 chain, four silanols/silanolates form a single four-member ring (S4R) that faces, slightly displaced, an identical S4R from the next chain, with hydrogen bonding along the [110] and directions providing connection between adjacent chains (Fig. ). Upon calcination, neighboring S4Rs in ZEO-2 connect to each other to form a D4R (Fig. ) by condensation of the terminal Si-OH groups, yielding the fully-connected framework of ZEO-3 (Fig. ). The condensed solid is a true, non-interrupted, three-dimensional extra-large pore zeolite. The channel system of ZEO-3 is 3D with 16×14×14 membered-ring (MR) channels (Fig. ) and full connectivity between channels (Fig. ). The structural models obtained were fully corroborated by Cs-corrected scanning transmission electron microscopy (STEM, Figs. ) where a faint signal corresponding to the tCyMP (C and P) was also identified within the 14 MRs of ZEO-2 (Fig. ).