The Blue Cave — Plava Špilja in Montenegrin — is carved into the limestone cliffs of the Luštica Peninsula, on the open-sea side near the entrance to the Bay of Kotor, in the Herceg Novi municipality. It sits about 18 km (around 40–60 minutes by fast boat) from Kotor Old Town, at GPS coordinates of roughly 42.374° N, 18.596° E. There's no road in — the cliffs drop straight into the sea, so the cave can only be reached by boat.
The cave has two entrances — a smaller opening on the south side and a larger one on the southwest side, once big enough for small boats to enter. The interior floor area is about 300 square metres, the sea inside is roughly 3–5 metres deep, and the vault rises about 9 metres above the water. It was formed over millennia by waves eroding the porous Luštica limestone.
What sets it apart from the famous Croatian and Maltese blue grottoes: it's far less crowded, far easier to reach from Kotor, and — uniquely among the big three — you can still swim inside. The water in the chamber is noticeably warmer and calmer than the open sea just outside. If you only have one morning in Boka Bay, this is the experience most first-time visitors remember.