General Information
It is first seen after one of the gun fights in Act 2. Later during the end of Act 2, It eats you when you are on top of a large house used as a landing zone for a Raven. By eating you and the rest of Delta Squad, it results in the chapter, "Intestinal Fortitude". during this chapter, your main goal is to find a way to kill the worm from the inside which Marcus states "then we'll rip its Goddamn heart out" and you will also try to find Benjamin Carmine. You have to dodge large obstacles, such as dodging the Riftworms grinding teeth, killing Nemcytes inside of it, and cutting its main arteries.
 About the rift worm
The worm itself is a very large, made out of a rocky type of structure. When the team realizes that they're inside the foul beast, they begin to panic and get a bit scared. Visually, inside, it may remind you of a cave, the walls are plastered with blood, muscle and living tissue. Be careful after the entry though, it may sound like a calm job but it is very dark for the majority of the mission, so look out for enemies. You must follow a few paths, or arteries in this case until you come to the large grinding teeth of the monster, just crouch and it shouldn't be a problem. Follow all paths until you reach a short cut-screen, after that short burst of excitement, a few enemies will shoot out of the ground of this beasts artery, simply just kill them, then walk along a hundred meters or so, until you see a figure of a man.
Yes, it is Benjamin Carmine. He has been severely attacked and is dying from multiple Nemacytes inside the beast, and unfortunately, he dies at this point. After this, you have to run away from a very large debris wall which was most likely started by when the city was swallowed. You have to shoot multiple flesh areas in order for it to open, and shooting trucks hanging from the ceilings to serve as a bridge and you continue through dodging the debris wall. You will know you have escaped it when it crashes into a wall that only you can go through. After chain sawing one of the main arteries, you can hear the worm move in pain and make a grunting noise and after this, you have to kill the worm by cutting all of the main arteries with a chain saw.
After cutting around 8 or so arteries, the game will come to a movie clip and show you cutting through its skin in order to leave the dead monster. But before this, after cutting one of the main arteries, you will have to go through a large room which is full of an acidic gas that will hurt you. Use your lancer at this point so when you find the wall you need to exit through, you can chain saw through it and you will not continue to choke on the gas.