My sister washed off some extremely muddy boots in my tub, and a good amount of mud, soil, and sand washed down the drain. Now it drains slow.

How do I clear it out?

(The plumbing for the tub is buried in the floor below).

Remove the tub over flow and plug with a wet cloth. Fill tub with about 6" of water. Plunge the drain. Plunge rapidly, you get more results from the pulling than the pushing. The plunging should break up and loosen the soil and mud in the trap. Let tub drain to wash away the mud and soil.

My sister washed off some extremely muddy boots in my tub, and a good amount of mud, soil, and sand washed down the drain. Now it drains slow.

How do I clear it out?

(The plumbing for the tub is buried in the floor below).

You can plunge as suggested,(see image) **OR**
You can snake the drain line thriugh the tub waste and overflow, (see image) Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. Now feed the cable( 1/4" best or 3/3" with a small tip. About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. Try plunging first. Good luck, Tom