The present island was formerly made up of several smaller islands, which were joined together during the 19th and early 20th centuries to form a single island. It is bounded on the north by Vasai Creek, on the northeast by the Ulhas River, on the east by Thane Creek and Bombay Harbour, and on the south and west by the Arabian Sea. The city of Mumbai lies on a peninsula on the southern end of the island, and Mumbai's suburbs cover much of the remainder of the island. The island contains Borivali National Park. It is also known as Sanjay Gandhi National Park. The city of Thane lies at the island's north-eastern corner, on Thane Creek. The coordinates are approximately 20° N and 72° E. Politically but most of the island falls into the jueisdiction of Muncipal Corporation of Greater Mumbai. It is divided into two separate districts, Mumbai City and Mumbai Suburbs. The northern portion of the island lies within Thane District, which extends across Vasai and Thane creeks onto the mainland.
The name Sashti means "sixty-six villages." Present-day Sashti Island was formerly several separate islands. Most of the northern and middle part of the present island were part of historic Sashti island, while the southern part of the island, which includes Mumbai City, was originally seven small islands (Mahim, Bombay, Mazagaon, Parel, Colaba, Little Colaba, and Sion), extending south from Sashti. The island of Trombay lays to the south-east of Sashti.
Coat of Arms of Bardez and Salsette under Portuguese rule.109 Buddhist caves, including those at Kanheri, can be found on the island, and date from the end of the 2nd century. The island was ruled by a succession of Hindu kingdoms, the last of which to rule the islands was the Silhara Dynasty. In 1343 the islands were annexed by the Muslim Sultanate of Gujarat. In 1534 the Portuguese took the islands from Sultan Bahadur Shah of Gujarat. Sashti was part of the northern province of Portuguese India, which was governed from Bacaím (now known as Vasai) on the north shore of Vasai Creek. In 1661 the seven Bombay islands were ceded to Britain as part of the dowry of Catherine of Braganca to King Charles II of England. Sashti remained in Portuguese hands. King Charles in turn leased the Bombay islets to the British East India Company in 1668 for £10 per year. The company found the deep harbour at Bombay eminently apposite, and the population rose from 10,000 in 1661 to 60,000 by 1675. In 1687, the East India Company transferred their headquarters there from Surat.
In 1737 Sashti was captured by the Marathas, and most of the Portuguese northern province was ceded to the Marathas in 1739. The British occupied Sashti in 1774, which was formally ceded to the East India Company in the 1782 through Treaty of Salbai.
In 1782 William Hornby, then Governor of Bombay Presidency, initiated the project of connecting the isles. The Hornby Vellard was the first of the engineering projects. The project gained momentum in 1817, and by 1845 the seven southern islands had been connected to form Old Bombay with an area of 435 km². Railway routes and road bridges were built in the 19th century to connect Bombay island to Sashti and vise-versa. These railway lines encouraged wealthier merchants to build villas on Sashti and by 1901 the population of Sashti rose to 146,993 and turned into Greater Bombay. The channels separating Bombay and Trombay islands from Sashti Island were filled in the early 20th century.
Parts of the island are hilly, although many of the hills were cut down and used to enlarge the island by filling in the shallows and to link the islands to one another. The highest point on the island is around 450 metres in the Borivali National Park, in the northern reaches of the island. This National Park is the world's biggest within city limits.
The island is located at the confluence of a number of fault lines. This makes the area earthquake prone, up to a magnitude of 6. The island is mostly composed of black basalt rock. Since it is along the sea coast, it has also a sandy belt on its western coast. The southern region of Old Bombay is mostly at sea level. However, the parts which were erstwhile shallows are below sea level. Many parts of the city are hilly. Also worth noticing is the occurrence of Laterite soil and rocks at a point on this island.
There are three major lakes on the island Powai Lake, Tulsi Lake and Vihar lake. The latter two lakes supply part of the city's water requirements. Numerous other smaller ponds and lakes are also present in the Thane region. On the shores of the Powai lake lies the celebrated Indian Institute of Technology Bombay.
Three small rivers, the Mithi (Mahim), Oshiwara and Dahisar, originate within the National Park, and empty into the Arabian Sea. The Mithi River originates from the Powai Lake. Vasai and Thane creeks are estuarine distributaries of the Ulhas River.
A number of saline or brackish creeks extend inland from the coastline. The Mahim creek separates the city from the suburbs in the west and the Sion Creek in the east (Sion creek no longer exists). Further north on the western coast, the Oshiwara river empties into the Malad (or Marve) Creek and the Dahisar River into the Gorai Creek. The eastern waterfront too, has many small creeks.
The small southern part of the eastern waterfront of the island forms the Bombay harbour. North of this region lie vast amounts of protected wetlands, home to migratory birds. The northern, north western part of the island and parts of Mahim River also have government protected marshlands. These swampy regions form massive and dense mangrove forests.
The western coast of Mumbai has numerous beaches. The best known is the Girgaum-Chowpatty Beach. Other well known beaches are Dadar Beach, Juhu Beach, Mahim Beach, Gorai Beach, Manori Beach and Worli Beach; of which Juhu, Manori and Gorai Beaches lies on west cost of Sashti island.