Qanat is an underground water catchment system that was developed 3,000 years ago in pre-Achaemenid Persia. The Qanat system is one of the most ecologically balanced water recovery methods available for arid regions. A qanat relies entirely on passive tapping of the water table by gravity. It has a very gentle slope that and rarely exceeds 5o/oo. A qanat may tap water either from an underground reservoir or through the microflows of the melting snow and rainwater seeping into the ground.
In modern Greece the biggest known qanat is that of Agia Paraskevi in Hortiatis, which is located at an altitude of 575–585 m at the west of the homonymous chapel. This qanat has been used for centuries to transfer water from the mountainous terrain to the hillside surface without need of pumping. Its water comes from an underground spring and through seeping precipitation. 
Additionally, an aqueduct was constructed to transfer the water harvested from the qanat and the springs of Hortiatis to the city of Thessaloniki. Surviving parts of this aqueduct testify to the total length of the construction reaching 20 km.  The aqueduct began in the exit point of the qanat and passed over an impressive water bridge that stands at the entrance of Hortiatis village. The impressive bridge used only for water transfer has a length of 223 m and a maximum height of 20 m. It was originally built in the roman era (1st cent. AD) and restored several times in the following centuries.
Then, the aqueduct mainly in the form of covered water channel, passed through the city of Panorama, outside the town of Asvestochori and reached Acropolis, from where the water entered the city of Thessaloniki and was distributed in public baths and fountains. This is the famous “hortaithen water”, mentioned in written sources since the 12th century. This water channel has not been altered for 1.500 years (!) supplying water to the city of Thessaloniki up until the year 1975. 
The subsoil in the area where the qanat was quarried consists mainly of recrystallised limestones of Triassic era and argilic shales of Jurassic era. The tunnel was opened into Quaternary sediments because digging in such a rock was technically easier. The presence of the permeable recrystallized limestone ensures the aquifer while the presence of a fault helps to increase the amount of water collected. The fault is no longer active.
The phyllites rock acts as an impermeable layer, and its upper surface is the basis of the qanat, thereby minimising water leakage. The water seeps from the karst aquifer and reaches the impermeable phyllites rock with a single possible way, the tunnel. 
The qanat is composed of two lines of tunnels with a total length of 74m, the one at a max depth of 6.40 m and the other at 7.90 m respectively. Water is drained over an area of approximately 600 square meters. Specifically, the qanat consists of :
- Two underground tunnels of total length 74 m, with average height 1.50 – 1.60 m and a width of 0.56 to 0.75 m. This opening allows workers to enter the tunnels for the qanat maintenance,
- Three shafts/ wells (1, 2, 3), two of which (1, 2) communicate with the surface, but the 3rd is closed.
- Four water reservoirs (α, β, γ, δ)
There are two main water entry points, where water spouts in the form of spring:
The first is located at reservoir (δ) where water spouts from the recrystallised limestone, follows the route of tunnels E, Γ, Β, and A and after reservoir a through a narrow pipeline reaches the end tank where it is chlorinated. Actually the whole tunnel E is dug on the interface between the permeable re-crystallised limestone on top the impermeable phylittes rock on the bottom.
The second water entry point also through recrystallised limestone is located at the base of the shaft (2) and following the route of tunnel Δ and then via pipeline arrives at the main collector.
Another source for karstic water is through seeping from the ceiling of the tunnels, as proven by the stalagmites of 2-3 cm found on their top. This water comes from the percolation of rainwater and snowmelt through the subsoil.
The entrance to the tunnels for humans at the level of the ground surface is located at reservoir (a). It should be noted that the water flows in a drain at the bottom of the tunnel. However the tunnel is opened at such a height so that workers can enter and walk through it for its maintenance. Parts of these drains have become today inactive due to deposition of calcium carbonate (CaCO3). [1, 2]
The hypsometric difference of the two tunnels (1,50 m) is the result of the manufacturers’ effort to increase the water supply by harvesting water at lower level (bottom of shaft 2). 
The Hortiatis qanat was not simply opened up into the subsoil, but in many parts it features masonry, consisting of plinths and rubble stones.
The average water flow of the qanat was approximately 22 m3/h back in 1994 , but according to more recent measurements it is 15 m3/h . The statistical analysis of the qanat discharge rate, for the period from 1966 to 1995, shows the fluctuation of discharge follows the fluctuation of rainfall in the area, with a relative delay. The strong correlation between qanat discharge and rainfall along with the geological composition of the drainage area suggest the qanat system collects water also from the karstic water table. 
Specifically, these tables show that the qanat discharge peaks in the months April – May – June, four months later than the peaking period of precipitation (December -January – February). It is concluded that the percolation of rainfall and snow melt in the karstic rocks of Hortiatis is approximately 4 months. This “delay” is attributed also to the climatic conditions in the Hortiatis Mountain that allow for a slow snowmelt throughout springtime.
The issue of dating the Hortiatis qanat is till today under question, due to the absence of written or any other form of evidence. According to Mr Vavliakis, the qanat was excavated during the Ottoman era, but Mr Manoledakis considers this estimate needs revision as the qanat is certainly earlier. 
Mr Manoledakis supports that the first construction phase goes back to late antiquity and is probably of the Roman era. He claims that during this phase bricks at least 4 cm thick have been used in the lower sections of the qanat where the water flows. He supports his view by drawing similarities between the cross section of the Hortiatis qanat with other known Roman qanats of the 2nd century AD. Furthermore, he attributes the construction of the aqueduct staring in Hortiatis village, which is of roman era, to the existence of the qanat.
In order to strengthen the qanat’s structure and perhaps to increase its efficiency, several sections of the wall were built with finest and often irregular bricks, still visible today dating from the Byzantine period.
Similar needs of improvement and expansion of the qanat were presented also later, during the Ottoman period. These are linked to Murat II, who carried out repair and maintenance works at the qanat and also constructed 20 new fountains in Thessaloniki, and the well known “Bath of Bei”.
Finally, it is known that the last major repair of the qanat took place in 1918 by the Thessaloniki Municipality. A few years later, in 1939, when the Thessaloniki Water Supply and Sewerage Company is established the qanat comes under its jurisdiction. In 1975, the qanat seized to supply water to the city of Thessaloniki but kept supplying water to a number of places like the Papanikolaou Hospital, a women’s Monastery outside the city of Panorama, a military camp in the same area and the Hortiatis village. 
Nowadays, according to the Thessaloniki Water Supply and Sewerage Company, the Hortiatis qanat supplies water only to the Papanikolaou Hospital.