How did we learn about the topography of the sea floor in the early part of the 20th century?

During the early 20th century, scientists learned about the seafloor topography through three primary methods: echo sounding, seismic reflection, and sampling and dredging. Echo sounding involved emitting sound waves from a ship to measure the time it takes for the waves to bounce off the seafloor and return to the surface, allowing for depth calculation. Seismic reflection provided more detailed information about layers beneath the seafloor by analyzing controlled shockwaves generated by underwater explosions or airgun shots. Sampling and dredging were accomplished by lowering weighted sampling equipment such as sediment corers and dredges, allowing for the physical collection and analysis of seafloor samples. These methods laid the foundation for modern oceanographic research, improving our understanding of the ocean floor's landscape and geological features.