Convert from zettameter to mark twain

Unit Definition (zettameter)
The zettametre (or zettameter), symbol: Zm, is an SI metric unit of length. 1 zettametre is equal to 1e+21 metres, a very large distance. Not to be confused with a zeptometre.

Unit Definition (mark twain)
The mark twain or twain is an old word for the number two, derived from the Anglo-Saxon twegen. The American author Samuel Clemens (1835-1910), who had been a riverboat pilot on the Mississippi in his youth, took his literary name from a traditional riverboat phrase "mark twain", meaning "exactly two" fathoms of water. This was the minimum depth needed for the boats to operate safely without running aground.