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1. The Gemara discusses people who offered a Korban Pesach and then discovered that they had made contact with a human corpse on the road.
2. The case above applies even when there is no possibility that they did not become Tamei.
3. It is called Tum'as ha'Tehom ("Tum'ah of the depths") because it must be covered in order to have that status.
4. If an entire Korban Pesach (or most of it) became Tamei, it is burned outside the Beis ha'Mikdash with wood purchased with public funds.
5. The Gemara explains why the entire Korban Pesach that became Tamei should be burned outside the Beis ha'Mikdash.
A BIT MORE
1. Since the corpse was not known to anyone when they came into contact with it, it is considered Tum'as ha'Tehom (see Daf 80, #4), and the Korban Pesach that they brought is considered valid, since the Tzitz atones for the Tum'ah.
2. In a case where there is a possibility that they did not become Tamei, they may even continue to eat Terumah, since the rule is that a doubtful case of Tum'ah that occurs in the public domain is deemed to be Tahor.
3. The corpse must be completely covered by earth, dirt, straw, or the like in order for the leniency of Tum'as ha'Tehom to apply to it. If it is between rocks or in water, it is not considered "in the depths," since it can be seen.
4. If only a small amount of a Korban Pesach became Tamei, it would be burned in private yards or on roofs using privately-owned wood. The same applies to meat from a Korban after the time allotted for its consumption has passed.
5. This is done in order to embarrass the people who allowed their entire Korban Pesach to become Tamei.
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