We study the triangular lattice Ising model with a finite number of vertically stacked layers and demonstrate a low temperature reentrance of two Berezinskii-Kosterlitz-Thouless transitions, which results in an extended disordered regime down to T = 0. Numerical results are complemented with the derivation of an effective low-temperature dimer theory. Contrary to order by disorder, we present a new scenario for fluctuation-induced ordering in frustrated spin systems. While short-range spin-spin correlations are enhanced by fluctuations, quasi-long-range ordering is precluded at low enough temperatures by proliferation of topological defects.PACS numbers: 64.60.F-, 05.50.+qIntroduction.-The antiferromagnetic triangular lattice Ising model (TLIM) is the paradigmatic example of geometric frustration [1][2][3]. Despite its simplicity, the TLIM exhibits all the defining features of a highly frustrated magnet. The extensive degeneracy of its ground state or Wannier manifold, which comprises any state without three parallel spins on the same triangle, leads to a residual entropy density S ≈ 0.323k B . This property makes the system very sensitive to perturbations, as is manifested in the algebraic spin-spin correlations. Simple perturbations, such as further-neighbor couplings, relieve the frustration and induce long-range order (LRO) or quasi-LRO [4][5][6][7][8]. The ground state degeneracy can also be lifted via the order by disorder mechanism [9]. For instance, a vertical 3D stacking of TLIMs produces a low-T partially disordered antiferromagnetic (PDA) phase consisting of two ordered sublattices with opposite magnetizations and the third one that remains disordered [10][11][12][13][14][15]. By adding a transverse field, we obtain the quantum Ising model (QIM) that also contains a low-T PDA phase stabilized by quantum fluctuations [16][17][18].In this Letter, we show an exotic classical spin liquid phase with unusual pseudocritical correlations in a simple generalization of the TLIM, namely, a vertically stacked finite number N z of triangular layers. This new phase consists of a line of pseudocritical disordered states. Surprisingly, spins become more correlated at short distances with increasing temperature: the spin correlation falls off like ∼r −η(T ) e −r/ξ with the exponentially large correlation length, ln ξ ∝ J/T , and the short-distance effective power law decay becomes slower at higher T (dη/dT < 0) [19]. This is similar to Villain's order by disorder [9]. However, while thermal fluctuations increase short-distance spin-spin correlations, hence the stiffness of the effective field theory, quasi-LRO sets in only when the stiffness reaches a critical value necessary to suppress the proliferation of topological defects.Our study is in part motivated by recent advances in filmgrowth techniques [20,21] and fabrication of artificial spin systems [22]. The model Hamiltonian is