Long Term Experiment is runs through a long period of timescales: a discrete experiment or a self-contained project constrained by the length of a funding cycle. Nature takes a look at five of science’s longest-running projects, some of which have been amassing data continuously for centuries. Some generate hundreds of papers a year; one produces a single data point per decade. Long-term experiments therefore contrast with nonexperimental long-term studies in which manipulation of the system studied is impossible. It designed to test a diverse set of hypotheses, with the expectation of a long-term funding commitment.