The significance of this place is that for a period of time in the history of this blessed ummah, the Juma prayer was only being offered in the Madina the Illuminated and the Masjid al-Jawatha, and nowhere else. Unfortunately not having access to a car during my travels in Saudi I was unable to visit this blessed place. May Allah (the Exalted) facilitate its visitation. Please find the hadith reference for this in Sahih al-Bukhari and a picture of its current condition:
[Note: At the time of the Prophet (Allah bless Him and give Him peace) the area of al-Ahsa was referred to as ‘Bahrain’, a common misconception to be avoided for those reading hadith literature who may think reference is being made to the small island which currently bears this name.]