Here is a suitable seed for a hardcore survival enviroment type of gameplay. Tall hills, extreme mountains and a couple of floating islands can be found in the seed.
It’s very easy to get there. Just take a turn to your right at spawn and you will see the first extreme mountains.
Here is one of the typical extreme mountains which can be found on several locations throughout the map.
In the mountain closest to spawn you can find a tunnel which leads into the mountain. It’s likely there is a more advanced tunnel system that can be accessed through here if you do some digging.