Dead zones are areas of the ocean where the water has low levels of oxygen, which can be suffocating for marine life. Ocean dead zones are increasing in size and frequency since the 1960s and could occupy around 20% of the world's oceans. These zones are caused by a variety of factors, including agricultural runoff, sewage discharge, and climate change. There are ways to prevent dead zones, including reducing agricultural runoff and increasing the number of wetlands.