Marble Caves of Puerto Tranquilo - Chile
At the base of the Patagonian Andes, the Cuevas de Mármol are some of the most beautiful cave formations in the world. There's a solid mile of formations to see along with a spectacular rock cathedral. One day I'd like to return with climbers to document some first ascents and deep water soloing.
The caves are a 20 minute boat ride from Puerto Rio Tranquilo and you can easily hire a local to take you to see the caves at nearly any time of the day. Although don't let the quick boat ride fool you. The town is a travel nightmare to get to. Unreliable public/private transport, an unpaved main highway, and the overall rural location all add to the difficulty.
The Carreterra Austral:
If you're at all planning on getting here its going to involve some if not all of the Carreterra Austral. Which is the main highway connecting the rural south of Chile to the rest of the country. If your planning to go this far it's a good idea to give yourself a month at the very minimum to tour the rest of Patagonia. The Carreterra Austral alone will take 1-2 weeks.
Puerto Rio Tranquilo has a small store offering just the basics. You'll find a solid meal but you wont find everything your looking for. The same with most of the journey. Make sure to carry cash too as theres multiple long distances without an ATM. Coyhaique and Chile Chico are great spots to stock up on anything needed.
The road is tough, and I wouldn't drive it myself without at least two spares. I have friends who hired a camper van in Punta Arenas and dropped it off in Puerto Montt and I'm sure you could do that in reverse too. But make sure you can take a rental across borders. I had another friend stopped at the border, unable to bring his Argentina rental into Chile. Personally, I think the best way to do it is just show up and be flexible. Ask around for (mini) bus times (a few a week) and hope it doesn't cancel, find/hire a local to drive you (easy), or stick out your thumb on the highway. I visited in the beginning of winter (June in the southern hemisphere). What it took for me to get here from El Chalten was a few full days of hitchhiking and trekking across the border in no mans land to Chile. I was alone and despite some long waits because of the empty roads, I felt I was moving pretty fast.
Camping, hostels, and cabanas with fireplaces are all available in Rio Tranquilo. I paid 10,000 Pesos a night sharing a cabana with four others and the owner provided firewood every day.
Overall its a spectacular location, surrounded by other spectacular locations (Tortel, Cerro Castillo, Cave of the Hands, Villa O'higgins, and El Chalten to name a few). I suggest making the trip to see it anytime between Sept-May.