Is the hole at the very bottom of the unit?
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Unfortunatly if the leak is coming from inside the unit (check to be sure it is not just running down the sides if the unit), at the bottom, the seals have gone out, and the unit will need to be replaced.
If it is leaking down the sides, you will need to determin exactly where it is comming from. You can do this by filling the sink with water and adding some food coloring to it. Red is best. Then drain the sink with the disposal turned on. The food coloring will show you exactly where the leak is.
If you have other concerns in this matter please reply here in this thread.
Best regards; The Home Smithy
Your units internal seals are bad. You will need a new unit as these are not repairable.
That hole with the square tab coming through it is the tab that holds the reset switch in place.That way you don't push it (the switch) into the disposal when you press it. (Trust me I have been an authorized ISE tech for 5 years) Smithy is absolutely correct about the leak. A Badger 5 costs about $60 the motor with seal for it runs about $50. It will take you about an hour and a half to replace it then more time to get it back in the sink. Replace the unit with a nice Sears (still an ISE) 3/4 HP stainless steel disposal with a 10 year in home warranty. It will run you $100 on sale. You will be able to dispose of just about anything that will fit in the sink hole including steak bones and corn cobs.
Please accept Smithys post, He has provided you with excellent advise.