  We study the quark mass dependence of the H-dibaryon in the strangeness
$S=-2$ baryon-baryon scattering. A low-energy effective field theory is used to
describe the coupled-channel scattering, in which the quark mass dependence is
incorporated so as to reproduce the lattice QCD data by the HAL QCD
collaboration in the SU(3) limit. We point out the existence of the
Castillejo-Dalitz-Dyson (CDD) pole in the $\Lambda\Lambda$ scattering amplitude
below the threshold in the SU(3) limit, which may cause the Ramsauer-Townsend
effect near the $N\Xi$ threshold at the physical point. The H-dibaryon is
unbound at the physical point, and a resonance appears just below the $N\Xi$
threshold. As a consequence of the coupled-channel dynamics, the pole
associated with the resonance is not continuously connected to the bound state
in the SU(3) limit. Through the extrapolation in quark masses, we show that the
unitary limit of the $\Lambda\Lambda$ scattering is achieved between the
physical point and the SU(3) limit. We discuss the possible realization of the
"H-matter" in the unphysical quark mass region.
