They got to know with the photographs from the U - 2 spy plane.
<em>Hope this helped! :)</em>
Answer
Explanation:
The 19th century began on January 1, 1801, and ended on December 31, 1900. The term is often used to refer to the 1800s, the century between January 1, 1800 and December 31, 1899. The 19th century was the ninth century of the 2nd millennium.
The people of the Indus Valley Civilization did not built massive monuments, like their contemporaries, nor they built rich among their dead in golden tombs, there where no mummies, no emperors, and no violent wars or bloddy battles in there territory. hoped it helped men <span />
