Seismic imaging was used to map the hidden bedrock valley under Hallsands beach in the UK. The researchers shot sound waves into the ground and measured how they bounced back to figure out the shape of the rocks below. They found two ancient river channels under the beach, one in the north and one in the south. The bedrock is covered by layers of gravel, peat, and muddy peat, with different speeds of sound for each type of material. The depth of the bedrock varies across the beach, with some areas having it closer to the surface than others.