We had heard that there was a smoking rock waterfalls on the border of Karnataka and Tamil Nadu. It is located 750 ft above sea level in the Melagiri Hills, Dharmapuri District of Tamil Nadu State, India.
There are 2 roads to Hogenekkal. One is ~200Kms with a clean road NH-7 i.e the Bangalore - Hosur - Krishnagiri - Dharmapuri (deviation here) - Hogenekal.
The other route which is ~140 Kms from Bangalore is through the forest reserve and is much shorter but, the road sucks and makes up abundantly with the views and quiteness of the forest. 
Keep asking for directions at these places - Bangalore - hosur road - attibele-dengeniKottay (deviation here) - Amchetty - Hogenekkal.
All along the route you will see giant deities, known as aiyanars, keepers of the peace in the village. They are made of terracota/plastered brick and colourfully painted. Sometimes, you see military soldiers standing guard to the terracota deities.
Granite rocks along the river which are 20 metres tall at some places.
There is an abundance of wildlife if you keep your eyes and ears open.
No oppurtunity lost. Boatmen sell chips and soft drinks on boats.
You see signs of Elephant posting on the road but you will be disappointed. The locals say there are elephants but deeper in the forest, shy of humans.
You pay a Rs 30 toll and receipt is given. Shame on this forest official who took another Rs 10 without receipt. Here is a photo of the man who did not give us a receipt. My way of protesting for receipt. Who knows if this is legal or illegal ?
Also local panchayat fleeces you again of Rs.20 as parking. The area comes under the jurisdiction of the Collector / Commissioner of Dharmapuri. :-). You can safely assume as to whom you are paying. 
The place is beautiful. Life is very evident here. You will be tormented by some boatmen trying to extract money.
The prices are very reasonbale and there are more than 500 licensed coracle operators. The boats are made of bamboo and black plastic and can be found all over South India. They are called coracles, butas, basket-boat, etc. Most of the boatmen are experienced and show a high degree of skill in navigating upstream. 
The stones are soft, shiny, hammered and pounded by years of water flowing. 
The boatmen promote the river with stories of movies being shot in such and such place. 
There are several places where fish is caught and cooked. As a blind rule a big fish is Rs 35 uncleaned. 
The women folk here sell fish in pieces cooked & spiced with chili/turmeric/salt/ginger/garlic at INR 4-10 a piece. A full cooked fish will cost INR 20.
There are lots of places where the Cauvery is shallow and several places where it is rocky and deep. Take care when diving and swimming. The locals will tell you where it is deep and where it is shallow. A very nice place to spend a day playing in the water, eating fish and soaking in the forest air. You could stay overnight too if you wished. There is Hotel Tamil Nadu. Detailed blog written by someone here.
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