In laymen’s terms it’s not to complex unless you try to make it so, every year thousands of tons of vegetable matter falls to the floor and is turned to mud by worms, anything that dies during that year also goes onto the mud and depending on many factors,ie oxygen,moisture, acidity, alkalinity etc depends on how it decomposes or in some cases is preserved, over many more thousands of years that will end up a long way down, no magic beings or conspiracy’s just plain and simple Mother Nature recycling everything and the earths crust getting deeper each year as a consequence