Islet at the mouth of the Brunei River
Chermin Island () is an islet at the mouth of Brunei River in the Mukim Kota Batu, Brunei–Muara District, Brunei. Historically, it played a crucial role in Brunei's defence strategy, particularly after the devastating raids during the Castilian War in the late 16th century. The island was fortified as a bastion to protect the capital and surrounding areas from further invasions, strategically positioned near the narrow channel upstream from the former capital. During a period of political and military instability, Chermin Island's significance was further underscored, serving as a key defensive point. It was later temporarily occupied by Sultan Abdul Hakkul Mubin as his palace following the assassination of Sultan Muhammad Ali, an event that sparked the Brunei civil war. 
Geography.
Chermin Island, located at the mouth of the Brunei River, spans and lies within the secondary bay southwest of Brunei Bay. Historically, it was a fortified location with defences dating back to the late 17th century, guarding the passage to the river alongside Kaingaran Island, situated approximately half a mile to the north. The bay's rugged escarpments and natural defensive features highlight its strategic significance.
Navigation around the island is aided by a red-and-white beacon positioned about northeast. The island's geographical features, complemented by strategically placed beacons and navigational channels, are vital for guiding vessels through the intricate waterways leading to the Brunei River. Due to its proximity to Brunei Town, Chermin Island is occasionally mistaken for part of the capital. The island, which rises to , is densely forested and characterised by steep terrain, dense jungle, and an abundance of crocodiles and snakes, making landing particularly difficult. Its northern extremity is marked by a prominent rock, with Chermin Rock, less than a fathom deep, situated nearby.
The island's vegetation includes nipa trees on the eastern side, linked to a mangrove area with species such as "Avicennia", "Rhizophora", and "Sonneratia". In 1990, the Universiti Brunei Darussalam (UBD) Biology Department recorded seagrass "Enhalus" here. The rest of the island features primary forest growth, with limited sandy beaches and mostly muddy shores. Significant erosion is evident on the western side of the island. Although currently abandoned, Chermin Island is occasionally used as a leisure spot by serious sports fishermen. 
History.
Ong Sum Ping moved to Brunei with his people and created Kota Batu, the sultans' palace, and built a stone bar at Chermin Island, strengthening the area's defensive qualities. Sultan Muhammad Hasan established his palace and tomb at Tanjong Kindana, at the entrance of the Brunei River. He fortified Chermin Island and constructed a bridge connecting his palace at Tanjong Kindana to the fort. Using cannons created during Sultan Shah Brunei's and his own rule, this was firmly enforced. "Pengiran Kesuma Negara" Pengiran Hashim states that 40 boats loaded with stones were sunk in order to begin building the stone fort between Chermin Island and Keingaran Island.
The administrative seat of Brunei was relocated from Kota Batu to Chermin Island, which is situated at the mouth of Brunei River, during the civil war (1661–1673) between Sultans Muhiyddin and Abdul Hakkul Mubin. A palace and several houses were proposed on the island by the "Pengiran Bendahara" and later ordered by the "Pehin Orang Kaya Digadong Seri" Nara. Mubin then relocated the administrative capital from Kota Batu to the island during his reign, while the "Pengiran Bendahara" did not. It was intended for Mubin to wait out the crisis taking place prior to the Brunei civil war, the relocation was under the guidance of the newly appointed Pengiran Bongsu (later Muhyiddin). After relocation to the island, Muhyiddin declared himself as the 14th Sultan of Brunei with his authority around Kampong Ayer thus sparking the only civil war in the country.
Mubin fought the war from Chermin Island, which he had fortified to protect himself. After breaking a treaty, Mubin ultimately made his way back to Chermin Island. With the aid of a troop from the sultan of Sulu, forces from Muhyiddin landed on the island, no fighting has taken place as most fortifications were destroyed by bombardment from Tanjong Kindana. The island's fort was taken, and Mubin was put to death. Mubin has sent most of the regalia into the Brunei Bay with cannons. It is claimed that he was buried on the island.
Sultan Muhammad Alam's death in 1828, marking the end of another civil war, is closely linked to Chermin Island, where he was allegedly assassinated by hired killers. However, historian Jamil Al-Sufri questions this account due to a lack of evidence. An alternative version suggests he voluntarily left Brunei and moved to Putatan in Sabah, where a gravestone is believed to mark his burial. While there is no proof he was buried on Chermin Island, the gravestone closely resembles other royal tombstones in Brunei.
The mouth of the Brunei River, between Chermin Island and Muara, was thoroughly surveyed in 1926 by a team from . On the evening of 27 December 1958, while traveling downstream from Brunei Town with a pilot on board, the SS "Juno" struck the turning light beacon off Chermin Island, causing the structure to collapse. In 1992, the island was studied by the UBD Biology Department, which recommended its preservation for its rich sediment, wildlife, and plant life. Additionally, the island has potential as an archaeological site for tourists if the historically significant fortifications once standing there are rebuilt. As of 2000, Chermin Island has been gazetted under the "Antiquities and Treasure Trove Act".
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