Unlike the canon counterpart, the ruins in Underfell are quite harsh and are filled with deadly traps and powerful enemies. They are also renamed to the Ravaged Mines.
Unlike the Ruins in Undertale, the Ruins in Underfell are completely red. The floor appears to be a dark shade of orange, with several cracks running down the walls. Several blood stains are found in the ruins at random locations, and the plates giving the player hints to solve the puzzles have been destroyed by Toriel. Instead, Flowey guides the player safely through the ruins. The spike set puzzle mid-way through also appear to be stained with blood. The dummy, appearing mid-way, appears beaten and torn with blood stains on it.
The player must complete each puzzle by themselves. The first puzzle at the beginning of the Ruins will impale the player if the wrong button is pressed. The dummy will be encountered mid-way through. Deadly enemies are found more commonly throughout the Ruins. The spike puzzle will kill the player if the wrong spike is pressed on. The player meets Toriel at the end of the long hallway. The monster candy is poisonous and will take away some of your health. The rock puzzle is hard since the rock is stubborn and will not move for you unless you fight it. The leaf puzzle has nails under the leaves. Napstablook will try to possess and kill you. Once you spare Napstablook, he will notice Flowey and ask him out on a date. Flowey, trying to act tough, blushes and leaves Frisk to date Napstablook. Frisk then needs to solve the puzzles on their own. The switch puzzle will kill you instantly and Toriel will lure you to her house. If you try to leave she will fight to protect you from King Asgore. Flowey returns at the end of the Toriel fight and goes to Snowdin with Frisk.
Instead of the rather calm and somewhat adventurous theme for the Ruins in Undertale, the Underfell theme emits a feeling of depression and despair.
Instead of a perfectly normal dummy, the one in these Ruins appears distressed, as several tear marks and holes line its body. The eye appears to have been ripped out, dangling by just a thread. There are blood stains all over it. The player has the option to spare the dummy.