Islands may disappear when the currents on the reef change. Maldivians call this phenomenon of erosion "giramun dhiyun". New islands also may appear, beginning as sandbanks or coral gravel heaps at another location of the reef (a phenomenon that is known among Maldivians as "vodemun dhiyun"). Therefore, in the Maldives, islands are constantly eroding and constantly being formed. Human action, in the form of jetties or the dredging of channels on the reef, may change the pattern of currents on the reef and accelerate erosion.