It is the cave where the Prophet, , and Abu Bakr, may Allah be pleased with him, lay hidden while they were on their way to Al-Madeenah during Hijrah (migration). They remained there until the Quraysh calmed down and gave up pursuing them and then they resumed their journey.
While the Prophet, , and his companion, may Allah be pleased with him, were in the cave, the Quraysh approached, looking for them until they reached the cave's entrance. In this grave situation, out of His bounty and limitless Omnipotence, Allah The Almighty safeguarded them. Abu Bakr, may Allah be pleased with him, said to the Prophet, , "If one of them looks down to his feet, he will see us." The Prophet, , however, assured him and said: "O Abu Bakr, what do think of two (persons) when Allah is their Third." [Al-Bukhari and Muslim]
Allah The Almighty recorded this incident in the Quran Saying (what means): {If you do not aid the Prophet -- Allah has already aided him when those who disbelieved had driven him out [of Makkah] as one of two, when they were in the cave and he said to his companion, "Do not grieve; indeed Allah Is with us." And Allah sent down His tranquility upon him and supported him with angels you did not see and made the word of those who disbelieved the lowest, while the word of Allah -- that is the highest. And Allah is Exalted in Might and Wise.} [Quran 9: 40]
The cave of Thawr is about four kilometers south of Makkah and Al-Masjid Al-Haraam (the Sacred Mosque). It is 748 meters above sea level. It is a hollow rock with a height of 1.25 meters. It has two entrances, one from the west through which the Prophet, and Abu Bakr, may Allah be pleased with him, entered, another from the east.
It is worth mentioning that there is no evidence in Sharee‘ah (Islamic legislation) that encourages visiting the cave of Thawr. Consequently, it is impermissible for a Muslim to visit the cave believing that there is a virtue for that visit in Sharee‘ah, let alone visiting it with the intention of seeking the blessing of its floor or stones. This is actually a prohibited innovation in religion.
Allah Knows best.