A sea cave is a big hole in a cliff near the ocean that goes all the way through the cliff and leads to the water.
Imagine you're playing with your toys on the beach and you see a big hill in the distance. You run over to it and see that it's made of big, tall rocks. You notice that one of the rocks has a huge hole in it, and you can hear the sound of water coming from inside.
You feel curious and decide to explore further. You enter the hole and find yourself in a dark and rocky tunnel. It's a little bit scary, but you see a tiny bit of light coming from the other end of the tunnel, so you keep moving forward.
As you get close to the end, you start to feel wet and hear the sound of waves crashing. All of a sudden, you see the beautiful blue ocean in front of you! But here's the thing – you're not outside, you're actually inside the cliff!
That's what a sea cave is. It's a dark and cool tunnel that leads to the ocean. And if you're lucky, you may even see some sea creatures in there, like crabs or starfish!