  GRAND is a newly proposed series of radio arrays with a combined area of
200,000 square km, to be deployed in mountainous areas. Its primary goal is to
measure cosmic ultra-high-energy tau-neutrinos (E>1 EeV), through the
interaction of these neutrinos in rock and the decay of the tau-lepton in the
atmosphere. This decay creates an air shower, whose properties can be inferred
from the radio signal it creates. The huge area of GRAND makes it the most
sensitive instrument proposed to date, ensured to measure neutrinos in all
reasonable models of cosmic ray production and propagation. At the same time,
GRAND will be a very versatile observatory with enormous exposure to
ultra-high-energy cosmic rays and photons. This talk covers the scientific
motivation, as well as the staged approach required in the R\&D stages to get
to a final design that will make the construction, deployment and operation of
this vast detector affordable.
