Smectic liquid crystals are unique condensed phases in that they exhibit properties of a one-dimensional solid and of a two-dimensional liquid. There exist various types of the hydrodynamic modes, propagating and diffusive modes, such as second sound or layer undulation modes, related to the existence of the internal degree of freedom in the complex fluid. Mechanical properties have been investigated of hyperswollen lyotropic smectic phases, with focus on its low dimensionality and large intrinsic fluctuations. This layer structure is supported by the microscopic long-range intermembrane interactions such as electrostatic or steric repulsion that play an important role in the hyperswollen regime. The layer compressibility measurements tell one that the layer structure is supported by entropic repulsion, which originated in the undulation fluctuation motion of the membranes. The light scattering experiments are also shown which provide information on the relation between the static structure and the dynamics of hydrodynamic fluctuation modes.