Relative sea level is the difference in height between the coast and sea surface. It changes across space and through time in response to many different physical processes operating on a range of scales. Using an example from North Carolina, USA, we introduce some of these key processes and explore how their importance varies on timescales from hours (e.g., hurricanes and tides) to millennia (e.g., ongoing subsidence of the coast). Below the salt marshes of North Carolina (and elsewhere) are sequences of sediment that that accumulated over thousands of years and record relative sea-level changes. Geologists collect this sediment, establish its age and interrogate it to understand when and how relative sea level changed long before people began to make systematic measurements. This research shows that the rate of rise during the twentieth century was without precedent in ~2000 years.