The Athenians now fell back to the island of Prosopitis in the Nile delta, where their ships were moored. There, Megabyzus laid siege to them for 18 months, until finally he was able to drain the river from around the island by digging canals, thus "joining the island to the mainland". In Thucydides's account the Persians then crossed over to the former island, and captured it. Only a few of the Athenian force, marching through Libya to Cyrene survived to return to Athens. In Diodorus's version, however, the draining of the river prompted the Egyptians (whom Thucydides does not mention) to defect and surrender to the Persians. The Persians, not wanting to sustain heavy casualties in attacking the Athenians, instead allowed them to depart freely to Cyrene, whence they returned to Athens. Since the defeat of the Egyptian expedition caused a genuine panic in Athens, including the relocation of the Delian treasury to Athens, Thucydides's version is probably more likely to be correct.