We report the structural transformation of hexagonal Ba 3 YIr 2 O 9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at ∼4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form γ T + βT 3 in the temperature (T ) range of 2-10 K with the Sommerfeld coefficient γ = 10 mJ/mol-Ir K 2 . The 89 Y NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba 3 YIr 2 O 9 . In this picture, the γ T term in the heat capacity and the linear variation of 89 Y 1/T 1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010) 9 It will be interesting to explore other iridates having a triangular lattice. Ba 3 YIr 2 O 9 has a similar chemical formula like Ba 3 IrTi 2 O 9 and it crystallizes in the hexagonal structure (P6 3 /mmc) with Ir-Ir structural dimers arranged in an edge-shared triangular fashion.10 Since all the Ir are equivalent, they should have a fractional oxidation state of +4.5 in a simple ionic picture. Our investigation of this 5d-based system is motivated by the fact that the fractional valence coupled with a geometrically frustrated lattice might lead to a spin-liquid state or possibly a heavy fermion state as in the 3d-based LiV 2 O 4 .11 This however did not turn out to be the case. Whereas we confirmed the onset of long-range order below 4 K in Ba 3 YIr 2 O 9 (in agreement with Ref. 10), we succeeded in suppressing the magnetic order with the application of pressure. In fact, when Ba 3 YIr 2 O 9 was subjected to a pressure of 8 GPa at 1273 K, it transformed to a cubic double perovskite structure as evidenced from x-ray diffraction under ambient conditions. Though the high-pressure (HP) synthesized sample remains insulating (based on our resistivity measurements), it has a metal-like linear heat capacity coefficient γ = 10 mJ/mol-Ir K 2 . Further, the 89 Y nuclear magnetic resonance (NMR) shift is found to be independent of temperature (T ) below 120 K and the 89 Y NMR spin-lattice relaxation rate crosses over from T -independent behavior at high temperature to a linear T dependence below about 45 K. These results point to the existence of low-energy excitations at low temperatures. In the absence of metallic behavior in the resistivity and the presence of local moments, the low-T data suggest the formation of an exotic ground state for the HP phase, possibly a gaple...