In shallow waters off the coast of Alabama, a well-preserved Ice Age bald cypress forest lies buried beneath the seafloor. The stumps, still rooted in place, grew more than 69,000 years ago when the sea level was much lower, exposing the seafloor where a coastal swamp began to grow. Our team mapped and sampled four sites using sonar, sediment cores, underwater vehicles, and laboratory analyses. We found that these trees lived in a low-oxygen wetland that allowed organic material to accumulate and be preserved even after rising sea levels from abrupt climate shifts. Fossil pollen, wood anatomy, and isotopes show that the swamp ecosystem changed with time —from cypress-tupelo forests to freshwater marshes, coastal wetland, saltwater marsh, and finally open ocean. Tree rings reveal a sudden die-off of the forest, possibly caused by rapid sea level rise or intense storms associated with abrupt climate change. This submerged forest provides a rare and valuable snapshot of what forests were present in the last glacial in the southeastern United States, and how the forest and coastal landscapes were affected by past climate shifts, which helps us understand how modern sea level rise and extreme weather might affect today’s shorelines and coastal forests.