These islands between Alibaug and Mumbai in the sea are very rocky and sea around them is dangerous.
On Khanderi there are two hillocks, the southern hill has one lighthouse which has been constructed by the British and it is still in use. Another hill which is on the northern side has one temple of Vetal. On the southern hill there is one rock which gives out metallic sound when struck. In the fort there is one old temple of Shiva. The bastions of the fort have some cannons on it. Three canons are with the carriages which were used to transport them. There is one well on the fort. A tomb of Doud is also there and worshipped by the local fishermen community.
Underi island has one cave with some carvings in it. The fort has on underground chamber which was used to keep prisoners. There are 15-16 canons on the fort.
Thal is a seaside fort in the Thal village and provides views of these two island forts. Its walls are still strong. It has one water tank in it. It is also known as Khubladha killa.

Past:
The earliest records of Khanderi and Underi islands are found in Portuguese documents in 1538 where the then viceroy Dorn Joao da Castro refer them as uninhabited islands. In 1674, an Italian traveller named Paschar visited this island. The fort Khanderi was built by Mainak Bhandari who was a Sardar in navy of Shivaji Maharaj to check the increasing powers of Siddi of Janjira and British in the North Konkan seas. It was completed in the year 1678. British forces aggravated by this move attacked Khanderi in the year 1679. In a series of fights between the two forces, British lost to Marathas on 11 November 1679 and the fort remained with them till 1775. In 1718, British again tried to capture the fort but their effort was not successful. Angre, naval chief of Marathas revolted against Peshavas and took control of this fort and both the other forts viz. Underi and Thal in 1799. Peshava forces regained it 1814. But they lost them to British in 1818 after their downfall.
Underi fort was built by Siddi Kasam in 1680 as part of the help to British forces who fighting with Marathas. Marathas although tried to regain this fort many times, succeeded only in 1760 when Peshava sardar Naro Trimbak defeated Siddi's forces. It is also called as Jaidurg

By public transport:

Thal is connected by direct ST buses from Mumbai. Another route from Mumbai is get launch from Gateway of India and reach Mandva and get down at Thal by the bus going to Alibaug. For coming from Pune and Kolhapur it is better to reach Alibuag and go to Thal by the bus. While coming from Southern Konkan and Goa reach Pen by ST, Private buses or train and then take bus which goes to Thal or go there via Alibuag.

Nearby attractions:
Varsoli beach
Kihim beach
Awas beach
Saswane beach
Mandawa beach
Aakshi beach
Nagoan beach
Alibag beach