Tuvalu consists of four reef islands and five atolls measuring 26 square kilometers, making it the fourth smallest sovereign nation in the world. With less than 11,000 inhabitants, only Vatican City is smaller by population.
With a maximum height of 4.5 m above sea level, only the Maldives has a lower maximum elevation. Tuvalu is extremely susceptible to rising sea levels, and it is estimated that an increase of 20-40 cm would make the islands uninhabitable.
Tuvalu on Wikipedia