First, start out by making a cube, and selecting a side to be the mouth, use ctrl to shrink the face, then again use it to make a hole in the cube. Then, shift select the face on the opposite side and press control again to make the cube like the one shown to the bottom. Then select an edge on the top of the cube/cave and press k to make an edge ring, and press connect. Then, selecting the top part of the loop you just made, move it upward in a more natural way, and do the same with the sides, keep on tweaking this until it looks right.(Remeber, it doesnt have to look exactly like mine, as all cave look different.) Then, selecting a face on the to or bottom, use control and make some stalagmites and stalagtites, or if you want to, select one on top, then the one underneath it, and press bridge, then do edge ring on it, and connect the edges, then, selecting the ring, shrink it to make a concave shape. once you are done tweaking it, smooth it to 3 (well, thats my preferance for my computer speed, so use your amount) and save it, then render it in your favorite program. Thank you for reading my tutorial!