This excerpt (conversation between Vyasa and Yudhishtira after Ghatotkach's death) is from Drona Parva of Mahabharata:
then the valiant son of Jamadagni, proceeding against the Kashmiras, the Daradas, the Kuntis, the Kshudrakas, the Malavas, the Angas, the Vangas, the Kalingas, the Videhas, the Tamraliptakas, the Rakshovahas, the Vitahotras, the Trigartas, the Martikavatas, counting by thousand, slew them all by means of his whetted shafts. Proceeding from province to province, fie thus slew thousands of crores of Kshatriyas.
Creating a deluge of blood and filling many lakes also with blood as red as Indrajopakas or the wild fruit called Vandujiva, and bringing all the eighteen islands (of which the earth is composed) under his subjection, that son of Bhrigu's race performed a hundred sacrifices of great merit, all of which he completed and in all of which the presents he made unto the Brahmanas were profuse.
It mentions that Earth is made up of 18 islands in 28th Dwapara.
What are those islands on the current world map and why can't we see them today(I want answer supported/cited from Scriptures please no scientific speculation). (i.e., 28th Kaliyuga of Vaivasvatha Manvantar) as it was back then (made up of 18 islands)?