The details of the topology are shown and discussed in the Supplementary Material (Tab. S9-S10). The ZEO-2 chain is topologically identical to the one found in polymorph B of zeolite Beta (although in that zeolite it is not an isolated chain but is embedded in the 3D framework). We point here that condensation of the chain found in polymorph A of the same zeolite (Fig. ) would result in a new chiral 16×14×14MR hypothetical zeolite. ZEO-3 and this hypothetical chiral zeolite correspond to the σ expansion of polymorphs B and A, respectively, of zeolite Beta . ZEO-3 is the first stable, fully-connected (alumino)silicate zeolite containing 3D interconnected pores opened only through extra-large windows (i.e. windows of more than 12 tetrahedra, >12MR). The crystallographic pore sizes of ZEO-3 are 10.36×8.51 Å and 9.79×8.00 Å for the 16MR and 14MR, respectively (Fig. ). The 3D extra-large pore nature of ZEO-3 translates into a very low framework density (FD) value (12.76 tetrahedral atoms, T-atoms, per 1000 Å 3 ). Compared with the other known stable, low density (alumino)silicate zeolites, including FAU, EMT, *BEA, BEC, ISV, IWV, and the recently reported PST-2, PST-32 , and ZEO-1 (5), this value is the lowest and puts ZEO-3 as the crystalline silica polymorph with the most open framework (Tab. S11). The calculated density of ZEO-3 is just 1.27 g/cm 3 , i.e. less than half that of quartz (2.65 g/cm 3 ) and actually much closer to the density of water. In fact, ZEO-3 breaks the observed tendency between the framework density and the size of the smallest rings in the zeolite structure . For an average smallest ring of 4.25, the predicted minimum FD ( ) is 13 T-atom per 1000 Å 3 , above ZEO-3's value. Compared with the real values of non-interrupted zeolites containing 4-and 5-rings, ZEO-3 is well below the lowest calculated FD of ISV and IWV (15.0; experimental values of 15.4 and 15.7, respectively). Additionally, ZEO-3 is significantly more stable than expected according to the known energy-density trend, as shown in Fig. , while the hypothetical σ-BEA, has the expected stability based on a calculation of framework energy after minimization (Fig. ).