This is the 10811th most frequent Turkish word.
denizaltı
Submarine.
The word 'denizaltı' is used here as a compound noun combining 'deniz' (sea) and 'altı' (below) to denote a submarine.
Denizaltı, suyun derinliklerinde yolculuk yapabilen bir araçtır.
A submarine is a vehicle that can travel in the depths of water.
In this sentence, 'denizaltı' is used to describe the area below the sea surface, translated as 'under the sea' or 'underwater'.
Balıklar denizaltında serbestçe yüzerler.
Fish swim freely under the sea.
Here, 'denizaltı' serves as an adjective to describe scenery related to the underwater environment.