Have you ever wondered how we know the depth of the ocean? Sonar is the main technology used investigate the location of submarine canyons, seamounts, abyssal plains and if the seafloor is sand or rock. This activity builds understanding of sonar technique and applications through a) processing and plotting provided data to discover a seafloor feature or shipwreck, and b) bouncing balls on different surfaces and analysing rebound, relating this to ground hardness then extend understanding to ‘backscatter’ in bathymetry surveys. It is suitable for middle secondary students, but could be extended for senior students of Earth science, marine science/oceanography, and physics.