Motility was one of the first accepted biosignatures. When Leeuwenhoek first observed bacteria and protozoa in the 17th century using single-lens microscopes, he knew they were alive because of their motion: “[N]o more pleasant sight has ever yet come before my eye than these many thousands of living creatures, seen all alive in a little drop of water, moving among one another, each several creature having its own proper motion” (Macnab, 1999). In Earth’s marine environments, even the most extremely cold or oligotrophic, many bacteria are motile or capable of motility. On icy worlds, it may not even be necessary to access subsurface oceans, as life—including organisms capable of motility—is present throughout Earth’s icy environments.