The monastery of Panagia Spiliani is built at the top of a hill northwest of Mandraki. Around the monastery retained sections
of the Paleokastro wall still exist, whereas the castle is protected by the «Castle of the Knights of Rhodes» built by the Knights of St. John in the early of 14th century. The biggest part of the monastery is built in a cave. The cave is divided into two churches. The north is dedicated to the Kimisi tis Theotokou (Assumption of Mary) and the south, Agios Charalambos (Saint Charalampos).
The wooden temple of the church is remarkable, and in some of its parts the original gilding is saved. Here we read the inscription «ΑΨΚΕ-1725» (APSKE΄-1725), the date of its manufacture. Inside the temple there are many built-in parts of older temples.
The Lord’s Table is based on a Roman altar, decorated with garlands. The Lord’s Table is part of the marble column with the inscription «ΑΡΙΣΤΩΝ … ΤΕΥΚΛΕΥΣ» (ARISTON … TEFKLEFS) Section of the old marble temple is saved probably from 10th-12th century.
Paleokastro is a classical period acropolis of the ancient city. It is still in very good condition because of its high quality construction and represents
one of the best fortification works of ancientry, basaltic andesite, one of the hardest rocks in the world. The main gate is composed by well-hewn stone blocks with a perfect fit of the connecting surfaces. The total length of the fortification is very big, as it surrounds the entire land-lubber site of the rocky hill above the south end of Mandraki. The wall and the cliff delimit an area of about 130 acres total perimeter almost 2 km, while the weight of some of the stones that form the wall, reach 3.5 tonnes.
The Museum is situated in Nikia, on a beneficial site above the edge of the caldera, allowing a panoramic view of caldera interior, the post-caldera domes and the hydrothermal craters as well. This traditional conurbation used to contain an abandoned school, now restored within the scope of a relevant project; today it accommodates the Museum facilities, following the required renovation and configuration works. The Museum has invigorated the conurbation and contributed to the overall tourist development of the island, providing the necessary infrastructure for the development of yet another type of alternative tourism.
The following thematic areas are covered in the main parts of the Museum:
1. Genesis and structure of volcanoes
2. Volcanic processes and products
3. Volcanoes in Greece
4. The Volcano of Nisyros
5. Other active volcanoes in Greece
6. Volcanoes, Environment and Man
Nisyros is one of the youngest large volcanic centres in Greece included in the active volcanoes along with Methana, Milos and Santorini.
The oldest rock formations are 160 000 years old, while the youngest (prehistoric) are 15 000 years old and cover the entire island. Most of Nisyros is mountainous, volcanic and very fertile. The island is composed of volcanic rock formations, with swelled elevations that were formed by previous volcanic activities. The various rock formations, (basalt etc.) have been used for the construction of various buildings such as the Palaiokastro and the bell tower of the Church of Saint John the Theologian just outside Nikea. The base of the island has been formed by lava. The rocks are covered by pyroclastic deposits and volcanic mud. Two possible eruptions occurred several millennia ago, around 25 000 years back. A large depression occurred in the eastern part half way across the island. The western crown of this depression forms the highest peak of the island. The peak of Saint John is in the eastern part of the depression and is about 100m above sea level. In 1956 schisms emitting smoke were noted in the ground next to the western and southern sides of Rammos. It is possible that the volcano erupted in 1442. In 1872, an eruption occurred that was accompanied by earthquakes, backfiring and red and yellow flames. Ash and other elements erupted and covered the ground of Rammos. During the eruption of 1873, a crater of about 6 to 7 metres was formed and ash and mud spewed out. The ground of the areas of Lakkoi and Rammos were transformed into a lake with hot salty water, the water that spilled out of the crater. The most recent violent eruption was in 1888 which formed a cylindrical aqueduct of volcanic elements with a diameter of at least 25 metres. The largest crater of the volcano, named Polivotis, has a diameter of 260m and a 30m depth. There are another 5 younger craters in the region, the largest of which is Stefanos with a diameter of 30m and a 30m depth. The volcano also surrounds a valley 2 400m in length and 950m wide.