Given the following context:

St Kilda (Scottish Gaelic: Hiort) is an isolated archipelago situated 64 kilometres (40 mi) west-northwest of North Uist, in the North Atlantic Ocean. It contains the westernmost islands of the Outer Hebrides of Scotland. The largest island is Hirta, whose sea cliffs are the highest in the United Kingdom.  Three other islands (Dùn, Soay and Boreray) were also used for grazing and seabird hunting. The islands are administratively a part of the Comhairle nan Eilean Siar local authority area.The origin of the name St Kilda is a matter of conjecture. The islands' human heritage includes numerous unique architectural features from the historic and prehistoric periods, although the earliest written records of island life date from the Late Middle Ages. The medieval village on Hirta was rebuilt in the 19th century, but illnesses brought by increased external contacts through tourism, and the upheaval of the First World War contributed to the island's evacuation in 1930. The story of St Kilda has attracted artistic interpretations, including Michael Powell's film The Edge of the World and an opera.Permanent habitation on the islands possibly extends back at least two millennia, the population probably never exceeding 180 (and certainly no more than 100 after 1851). The entire remaining population was evacuated from Hirta (the only inhabited island) in 1930. The islands house a unique form of stone structure known as cleitean. A cleit is a stone storage hut or bothy; whilst many are still to be found, they are slowly falling into disrepair. There are known to be 1,260 cleitean on Hirta and a further 170 on the other group islands. Currently, the only year-round residents are military personnel; a variety of conservation workers, volunteers and scientists spend time there in the summer months.The entire archipelago is owned by the National Trust for Scotland. It became one of Scotland's six World Heritage Sites in 1986, and is one of the few in the world to hold mixed status for both its natural and cultural qualities....

answer the following question:

What is slowly falling into disrepair? 

cleitean