The film starts out with a shot of a plaque left on the moon to commemorate the first moonwalk in 1969—you know, that huge step for mankind.

Well, that whole history takes on an ominous undertone when something passes over the plaque, casting a shadow and disturbing the moon sand so much that the astronauts' footprints disappear.

By showing the erasure and/or overshadowing these records of the U.S. moon landing—which itself is a huge symbol of human achievement—Emmerich clues us into the fact that something or someone is threatening to erase/overshadow humanity itself.