by the threat of current and future sea level rise. A warning system, which uses the Iridium satellite network, was introduced in 2016 to allow outlying islands to be better prepared for natural disasters. The highest elevation is above sea level on Niulakita, Tuvalu thus has the second-lowest maximum elevation of any country (after the Maldives). The highest elevations are typically in narrow storm dunes on the ocean side of the islands which are prone to overtopping in tropical cyclones, as occurred with Cyclone Bebe, which was a very early-season storm that passed through the Tuvaluan atolls in October 1972. Cyclone Bebe submerged Funafuti, eliminating 90% of structures on the island. Sources of drinking water were contaminated as a result of the system's storm surge and the flooding of the sources of fresh water. George Westbrook, a trader on Funafuti, recorded a cyclone that struck Funafuti on 23–24 December 1883. A cyclone struck Nukulaelae on 17–18 March 1886. A cyclone caused severe damage to the islands in 1894. Tuvalu experienced an average of three cyclones per decade between the 1940s and 1970s; however, eight occurred in the 1980s. The impact of individual cyclones is subject to variables including the force of the winds and also whether a cyclone coincides with high tides. Funafuti's Tepuka Vili Vili islet was devastated by Cyclone Meli in 1979, with all its vegetation and most of its sand swept away during the cyclone. Along with a tropical depression that affected the islands a few days later, Severe Tropical Cyclone Ofa had a major impact on Tuvalu with most islands reporting damage to vegetation and crops. Cyclone Gavin was first identified during 2 March 1997, and was the first of three tropical cyclones to affect Tuvalu during the 1996–97 cyclone season, with Cyclones Hina and Keli following later in the season. In March 2015, the winds and storm surge created by Cyclone Pam resulted in waves of to breaking over the reef of the outer islands, causing damage to houses, crops and infrastructure. A state of emergency was declared. On Nui, the sources of fresh water were destroyed or contaminated. The flooding in Nui and Nukufetau caused many families to shelter in evacuation centres or with other families. Nui suffered the most damage of the three central islands (Nui, Nukufetau and Vaitupu); with both Nui and Nukufetau suffering the loss of 90% of the crops. Of