There are two ways to get there. Walking (Overworld or not), or through the hidden underground passage in the northwest of Joppa.
Depth and ConnectionsEdit
Red Rock is 4 levels deep.
A river always spawns on level 4, running south. Follow it to access the Waterlogged Tunnel, from which you can enter the expansive Underground biome or reach Joppa by following the river south for ~6 screens.
The Girshlings on the bottom floor can be dangerous at lower levels. Level 4+ is suggested, and still be careful of drawing more than one at a time.
You can find a hidden passage to the Waterlogged Tunnel, which connects directly to Red Rock, by using the search command (S key) near the 2x2 square of water in the northwest corner in Joppa. This route is about as dangerous as traveling above ground. I find the underground route a little safer, since there isn't a chance of getting attacked by a Salthopper.
Flora and FaunaEdit
Red Rock seems to have the same spawning rules as the Underground biome.
Plus a small number of: