When the World Shook — H. Rider Haggard

If representatives of an advanced civilization were to visit our planet today, would they be impressed or dismayed by the way we live?
When three adventurers, Bastin, Bickley and Arbuthnot, are marooned on a South Sea island, they discover an ancient crystal sepulchre. Inside are two Atlanteans who have been in a state of suspended animation for 250,000 years
One of the awakened sleepers, the haughty Lord Oro, is the last of the Sons of Wisdom, a superior race who'd relied on their advanced technology to subjugate the planet's lesser peoples. The other Atlantean is Oro's daughter, Yva, heiress to the title of Queen of the Earth... who falls in love with Arbuthnot.
Using astral projection, Lord Oro visits London and the battlefields of the Western Front. Unimpressed with the state of the world in the early 20th century, he sets out to do what he's apparently done once before -- use a colossal gyroscope to drown the planet, and restart the course of human history.
A darkly humorous look at the politics and conflicts of his own era by an author best known for swashbuckling adventure novels (including the hugely popular King Solomon's Mines) set in the context of the Scramble for Africa.