The formation of giant shallow craters was found on the surface of lithium triborate single crystals after the bombardment by argon cluster ions. The diameter of these craters varied from a few dozen nanometers to 500 nm that is more than an order of magnitude greater than the diameter of common gas cluster ion craters. As distinct from the craters produced by an individual cluster ion impact, anomalous ones were revealed only at the high ion fluence (i.e. at multiple overlapped cluster impacts), both in the high and low incident energy-per-atom bombardment modes (E/N = 105 and 10 eV/atom, respectively). Such anomalous sputtering can be a result of a high local energy release in the cluster-solid collision spots coupled with the presence of water molecules in the subsurface layer of hygroscopic borate-base single crystals.