The Beneath is completely made out of pure stone and the only way of getting in or out is through its ceiling, aka Overworld's 0 layer.
Caves, caverns and chasms are the norm, and they go all the way down from the top. Across the more open areas it is possible to find long forgotten fortresses, host to hordes of monsters.
Most overworld minerals and ores
Viridium: Very lightweight and somewhat common
Mythril: Arcane and malleableIt should be noted that coal can't be found in the Beneath as the depth pressurizes carbon so intensely that only diamonds form.
First and foremost, staying in low level light for more than a few seconds will harm you, as the darkness here is not the same of other places. That doesn't make common visibility problems any less dangerous, +50 blocks falls aren't uncommon and it can be pretty difficult to get back to the top. Players also need to be wary of shadowy hands that'll try to bring them into dark areas.
All mobs that spawn in the beneath have increased damage and HP.
Recovered from the torn page of a journal:
I never should've come here, but I sensed something calling out to me. After finding so much worthy ore near the bedrock I thought that going deeper would be a good idea. Oh how wrong I was, I've survived to countless creepers and skeletons, I've even faced a cave troll and manage to make it alive. Beneath is different, nasty creatures try to swarm me and even the weakest common monsters seem stronger.
Am I going insane? I just want to go home with my husband and our child, but I was too stupid, too greedy. I hear their voices, I'm scared, this place seems to be toying with me and I hear things near, crawling behind the walls, for once, I'm afraid of the dark. It never had been a problem above, just place a torch and you are safe, but torches won't save you here, each time that the darkness seems to grow closer I feel my energy slowly slipping away, I'm tired.
My only hope is that someone would find my diary and pass on the message, the message that the stories about The Beneath are true. I can't see my feet and I can barely read what I'm writing, gi-..."
This isn't your average everyday darkness... This is... Advanced darkness.
I've been told that at a certain depth under the ground, diamonds become more and more common, to the point you can no longer find coal but only diamonds and even other materials unknown to most people.