Borivali National Park is the only Wild Life Sanctuary in the world, inside a Metropolitan city limits. Spread across an area of 110 sqkm, it is home to rich flora and fauna. Among the variety of animals found here includes Panthers. Some parts of the forests are deciduous, some are semi-evergreen and some evergreen. The evergreen forests remind the rainforests in the southern parts of the Western ghats. Monsoon and immediately after monsoon are the ideal time for trekking. The Kanheri caves dating back to 4th century is within the forest limits (We did not cover Kanheri in this trek).
For more on BNP check-out, http://members.tripod.com/~KSHROFF/, http://www.indiawildliferesorts.com/wildlife-sanctuaries/sanjay-gandhi-sanctuary.html