In the Ramayana, Lord Ram became thirsty on the way to Lanka to rescue Sita. There was no water, so he shot an arrow into the ground, and the Ganga River sprang up from it. Mumbai's Banganga Tank means "Arrow Tank," and that is its source. (You can see it in the bottom left corner).
The tank is surrounded by temples, including a Shiva Temple. It is at the site where Lord Ram wished to pray to Lord Shiva before heading to Lanka. A linga wasn't available, so Lord Ram made one out of sand.
There is also a Kali Temple, partway up a steep hill.