Preheat your oven to 350°F.
In a large skillet over medium heat, cook the ground beef until browned. Drain any excess fat.
Stir in the taco seasoning and water, cooking for an additional 5 minutes until the mixture thickens.
In a large mixing bowl, combine the cooked beef mixture, black beans, corn, diced tomatoes, and half of the crushed Doritos. Mix well.
In a greased 9x13 inch baking dish, layer half of the beef mixture, followed by half of the sour cream, and half of the cheddar cheese. Repeat the layers with the remaining ingredients.
Top the casserole with the remaining crushed Doritos and the rest of the cheddar cheese.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the casserole cool for 5-10 minutes before serving. Garnish with chopped green onions and black olives, if desired.