Science fiction novel by David Weber. The plot revolves around Dahak, a stupendously large spaceship which arrived in the solar system nearly sixty millennia ago. It has remained undetected only by having disguised itself as the Moon. No typo there.
Dahak is, actually, not just a spacecraft, but a space battleship of the so-called Fourth Imperium sent out to the solar system to secure the Imperium's borders against the expected incursion of the Achuultani, some terrible alien race which has nearly extincted the human race many times before (which is the reason there have been so many past Imperia). However, the Imperium has been resurrected each time so far, and it has seeded many planets sterilised by the Achuultani with human life.
However, the human species did not descend from those seeds, but actually from sailors aboard Dahak which had to escape when its captain tried to resolve a mutiny by contaminating the ship, rendering it uninhabitable. The loyalists escaped in lifeboats and were hunted down and reduced into stone-age humanity by the mutineers which have been secretly involved in human history, operating from their secret base underneath Antarctica. There is, however, a renegade faction of `double mutineers', fighting the 'Southerners' all through human history.
When Dahak is accidentally discovered in 2039 by a US Navy space pilot, he is captured by it and, since he is one of his descendants, becomes Dahak's new captain. Subsequently, all hell breaks loose.
This is a fast-paced, action-packed novel, however it's rather entertaining and it's not really the cheesy space opera one would expect from the plot summary.