every three weeks, about seventy percent of guanabara bay's water gets flushed into the ocean, yet this is not sufficient to balance the pollution due to untreated discharges.

with a surface area of nearly 400 square kilometers, the bay has a 130 km perimeter and measures between 28 to 30 km form sides to sides, with the entrance into bay facing the ocean and 1700 meters wide.

the bay formed at the end of the last ice age, when the ice sheet melted and left water in its place, causing the bottom of the bay to be muddy, as it remains to this day.