Normally, it’s all but impossible to hear underwater sounds from above the water surface due to reflection at the interface. By placing a novel metamaterial in contact with the surface, however, Sam Lee of Yonsei University in Seoul and colleagues show that the structure enhances sound by a factor of 160 and allows 30% of the sound energy to get through. The metamaterial is a cylindrical shell with a thin plastic membrane divided into segments and with a mass in the centre, and was designed to respond to sound with secondary waves that interfere destructively, boosting transmission. Possible applications range from better underwater microphones to non-contact ultrasonic imaging.

Further reading
E Bok et al. 2018 Phys. Rev. Lett. 120 044302.