  Application of transformation theory to underwater acoustics has been a
challenging task because highly anisotropic density is unachievable in water. A
possible strategy is to exploit anisotropic modulus rather than density, while
has not been experimentally demonstrated. We present an annular underwater
acoustic cloak designed from particular graded solid microstructures. The
geometry tailored microstructures mimics meta-fluid with highly anisotropic
modulus through substantially suppressed shear wave. Transient wave experiments
are conducted with the cloak in a designed 2D underwater waveguide system and
proved excellent cloaking performance for enclosed target over broadband
frequency 9-15 kHz. This finding paves the way for controlling underwater
acoustics using the structured anisotropic modulus meta-fluid.
