The opaqueness of the canal's water obstructs sunlight to one-third of the depth needed for the bottom-level growth of aquatic plants. Rising gas bubbles betray the decomposition of sewage sludge that on a warm, sultry day produces the Gowanus Canal's notable ripe stench. One reporter described the smell as "like sticking your head into a rubber boot filled with used motor oil and rotten eggs," while another said that it was "less a scent than an assault that reaches in to choke the throat. Sometimes it has the biting odor of petroleum, with more than a hint of dead fish." There are reports that the smell has lessened in recent years as oxygen levels in the water have increased.