Underwater rivers and lakes exist, formed by saltwater mixing with hydrogen sulfide. This mixture is heavier than seawater, causing it to sink and create the appearance of flowing water. These underwater bodies can be found in locations like the cenotes in Mexico and some deep-sea lakes. While some areas can be toxic due to hydrogen sulfide, others, called “cold seeps,” have rich ecosystems and support marine life. Scuba divers can capture stunning images of these unique underwater phenomena.