A few years in the future, the icecaps melts, sea levels rose and southern tectonic plates shifted upward exposing the landmass of the Antarctic.
The ocean rise floods vast tracts of land prompting a move to floating cities made from wood. A new icecap forms over Canada. Much of the Mississippi valley is shallow swamp. In an effort to reduce atmospheric CO2, fast growing trees are planted and the wood is converted to boats. Most energy systems are converted to battery and electric motors recharged by non-hydrocarbon fuels.
A few years in the future, the icecaps melts, sea levels rose and southern tectonic plates shifted upward exposing the landmass of the Antarctic. Most of the Pacific islands in the South Seas fell below water level. But in Polynesia, specifically the Cook Islands, Tongavera atoll, rises to reveal a small central island surrounded by reef. The sharks have left and been replaced by dolphins.
An overflight finds the new South Pacific Atoll and the US military send geographers and scientists to explore the new atoll. They find the ruins of pyramidal stone buildings with smooth walls and flat roofs. On the east side of the central island there are a a dozen black stone altars with a depression in the anatomically correct outline of a man and on the west side of the island are another dozen black stone altars with dolphin shaped depressions.
There are steles with pictograms reminiscent of Egyptian and Sumerian writing. That puts them circa 2800 BC. Translated they tell the story of a civilization that was destroyed by some extraterrestrial object. No one can complete the translation of the texts. The object is described as the harbinger of death from above. The ancients could see it coming and their shamans and magicians created two islands--one for men and one for women, with sexually distinguishable altars in order to avoid destruction. Sonar locates the second island still at the bottom of the ocean. The Brass and intelligentsia believes that the altars were for sacrifices -- human and dolphin. They also assume that the inhabitants were wiped out in a meteor strike.
All of that is reported back to headquarters. Then the expedition disappears completely. A second group is sent to find out what happened.