Complex sentence: In the explored part of the cave there are several dozens of grottoes; the largest one, which is called the Druzhba (Friendship) Grotto, was given its name in honour of the participants of the International Geological Congress who visited the cave in 1937.
Split into simpler sentences: In the explored part of the cave there are several dozens of grottoes. The largest one, which is called the Druzhba (Friendship) Grotto, was given its name in honour of the participants of the International Geological Congress who visited the cave in 1937.