Strange underground cavern with signing in the Unicorn Tavern at Ancient Spire Outpost. Could use feedback.

Okay, so I was just playing, and loaded in with a Galleon and spawned on Ancient Spire Outpost in the tavern, as you normally do. I noticed something odd, in that there was a great big hole in the floor to the left of the bar, so naturally, I followed them down. It led into a very small cavern, maybe only half the size of the tavern itself. Inside, there was a small campfire and a backpack leaning against a rock near it. To the far end, there was a waterfall that came from the roof and hit the ground, vanishing. All around the cavern were weird glowing mushrooms. Behind the waterfall was a glowing part of the rock, maybe half the size of a character. From what seemed like behind the waterfall was singing. It was distorted, but I could make out female voices singing the song "Drunken Sailor" with creepy whispers behind it. So, I left the cavern, really curious, and exited the tavern to see where I was. When I turned around to head back into the hole, it was completely covered with stone. Naturally, I immediately took to both Reddit and the Sea of Thieves Fourms to check for any information. I found nothing. I have no idea if this is a common event of the game (I am still fairly new), of if it's something really rare and special. Love to hear what you have to say. Thanks!