  We discuss the possibility of performing blind surveys to detect large-scale
features of the universe using 21cm emission. Using instruments with approx.
5'-10' resolution currently in the planning stage, it should be possible to
detect virialized galaxy clusters at intermediate redshifts using the combined
emission from their constituent galaxies, as well as less overdense structures,
such as proto-clusters and the `cosmic web', at higher redshifts. Using
semi-analytic methods we compute the number of virialized objects and those at
turnaround which might be detected by such surveys. We find a surprisingly
large number of objects might be detected even using small (approx. 5%)
bandwidths and elaborate on some issues pertinent to optimising the design of
the instrument and the survey strategy. The main uncertainty is the fraction of
neutral gas relative to the total dark matter within the object. We discuss
this issue in the context of the observations which are currently available.
