Underground, tar may survive for a long time if it is sheltered from sunlight and abrasion, but soil moisture, microbes, pressure, and contamination can slowly change it.
Tar underground does not weather in the same way as tar in sunlight. It is protected from ultraviolet light and surface abrasion, so some residues can survive for decades or far longer. That is one reason tar-like archaeological materials can sometimes be recovered from buried contexts.
Soil still affects tar. Moisture can move around it, mineral particles can become embedded in it, and organic acids or microbial activity may alter the surrounding material. The tar may become harder, dirtier, more brittle, or more mixed with soil over time.
Buried timber treated with tar is a complicated case. Tar can slow water entry into the wood surface, but underground posts often fail where moisture, oxygen, and decay organisms remain active. If water is trapped at the soil line, tar alone cannot guarantee protection.
Underground tar should be thought of as persistent rather than invincible. It resists many forms of decay, especially compared with bare organic material, but it still changes according to soil, moisture, temperature, and time.