Covering over 70% of the Earth’s surface, the undersea landscape or bathymetry has been revealed only within the past century due to advances in technology for measuring the depth of the ocean. Current depth measuring techniques employ vehicles ranging from near-bottom remotely operated vehicles to ships on the sea surface to satellites high above the Earth. Depending on vehicle and mission, these systems utilize acoustics, optics, or radar altimetry to either directly measure or infer bathymetry. Each method provides different spatial resolution and can probe depths ranging from shallow coastlines to the deepest trenches. Coastal estuaries and bays must also be evaluated in terms of tidal currents that can change bathymetry on hourly timescales. Improvements are continually made to the types of data and analysis methods used for estimating bathymetry across the global ocean.