Twice in deep time, shifts in Earth's orbit strengthened the African monsoon and pushed its rain bands north across the Arabian Peninsula. The first pulse, roughly 100,000 and 80,000 years ago, filled the Mundafan depression to depths of 30 meters, covering at minimum 100 to 210 square kilometers of basin floor. The second came during the early Holocene, between 9,000 and 6,000 years ago, producing a shallower but still substantial lake of perhaps 58 square kilometers. Reeds and sedges fringed the water. Riparian forests and savanna spread outward from the margins, punctuated by occasional wildfires. In and around the lake lived mussels, snails, sponges, and fish, while the grasslands supported a menagerie that reads like an African wildlife census transported to Arabia: aurochs, gazelles, hartebeest, hippopotamuses, ostriches, water buffalo, wild horses, and wild asses. The record of these wet periods survives not only in Mundafan's sediments but in the stalagmites of caves in Oman and Yemen, which grew layer by layer as dripping water carried the chemistry of wetter times underground.

Mundafan formed in a topographic depression roughly 363 square kilometers across, hollowed out by wind deflation over geological time. To the east, the Tuwaiq Escarpment, a Jurassic limestone ridge, provided a natural boundary. The lake itself occupied a former riverbed that had been dammed by migrating dunes, or perhaps by faulting in the bedrock. When water levels rose high enough, the lake may have overflowed northward. Its catchment area was large relative to similar basins on the peninsula, which explains why Mundafan's lakes lasted longer and left thicker sediment deposits than any other former lake in Arabia. Those deposits, reaching 24 meters in places, tell the story in layers of clay, marl, and silt. Some researchers initially questioned whether Mundafan held true lakes or merely wetlands, since the wind has scoured away most of the characteristic shoreline landforms. But fossil evidence, including the freshwater mussels and snails preserved in the sediments, points clearly to standing open water deep enough to sustain aquatic life for centuries at a stretch.

Archaeological sites ranging from the Middle Paleolithic to the Neolithic cluster around Mundafan, often on or near its former shorelines. Some sites sit well into the lake's interior, suggesting that people occupied the basin even during lower water levels, following the retreating edge. These were not settled communities. The people at Mundafan were mobile hunters who moved with the seasons and the game, carrying obsidian traded from Yemen far to the south. At site MDF-61, near the lake's southwestern end, easy access to water and game made it a favored camp for an extraordinarily long period, leaving behind dense accumulations of stone tools, including arrowheads with striking resemblances to types found in the Levant and in Africa. This is the detail that electrified researchers when Mundafan came under serious scientific scrutiny in the 2010s. The tools, the trade connections, and the sheer persistence of human habitation demonstrate that Homo sapiens was present in the interior of Arabia during wet periods, supporting the theory that monsoon-driven green corridors through the desert served as migration routes during humanity's pivotal expansion out of Africa.

Today the Mundafan basin is one of the most inhospitable places on Earth, which is precisely why its scientific study started late. Fieldwork in temperatures that routinely exceed 50 degrees Celsius, in terrain with no water, no shade, and no roads, discouraged early investigation. But climate scientists and archaeologists who finally reached the site found a palimpsest of extraordinary richness: lake sediments recording monsoon cycles, fossils cataloguing entire ecosystems, and stone tools tracing the movements of early humans across a landscape unrecognizably different from the one visible now. From the air, Mundafan is a subtle feature, a slight depression in the rolling dune fields, its clay outcrops the only suggestion that water once collected here in quantities large enough to support hippopotamuses. The contrast between what was and what is captures something essential about the Arabian Peninsula: this is a land where climate has rewritten the landscape so completely that only the buried evidence remains to tell its older stories.