Depends on the box, it could be gravel underneath but it I would bet is a solid concrete box with pipes coming in at the top and a drain pipe below that is supposed to take all the water away. Once you clear it out you will see what you are dealing with. The similar setup at my house has 3 perimeter drain lines coming in, and one drain line that goes to the street sewer. Every house is difficult. Take a picture when you have it cleaned out so you can refer back to it next time.