http://news.bbc.co.uk/2/hi/science/nature/4739741.stm

Key excerpts:

Hurricane Ivan generated a wave more than 90 feet (27 metres) high -
thought to be the tallest and most intense ever measured

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The distance between the crest of the biggest wave and its trough was 91
ft (27.7 metres) but they suspect the instruments missed some waves that
were as tall as 132 ft (40 metres).