The SI prefix "mega" represents a factor of
10^{6}, or in exponential notation, 1E6.

So 1 megatonne = 10^{6} tonnes.

The definition of a tonne is as follows:

A tonne (also called metric ton) is a non-SI unit of mass, accepted for use with SI, defined as: 1 tonne = 1000 kg (= 10^{6} g).

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