The grotto shouldn't be considered a feature because it's just some void pits, as well as small bricks coming out of the wall. However, due to its ability to access multiple areas in the game with easy glitching, namely the River Tunnel and the Swamp Rock, it seems logical for quite a few others to know. It is NOT recommended to visit the grotto as there is no way out, and you will lose your axes if you go down there - as you will most likely have to reset.
The grotto also has a series of walls that you have to somehow get around, allowing you passage all across the Swamp and above the River Tunnel. It circles the Pink Block (Swamp Rock) and makes a dead end straight above the entrance to the tunnel.
Accessing The Grotto
As of 7/2/16, three anonymous users accessed the grotto by glitching with a vehicle, and as of late, this is one of the known ways to get inside (there is more to be discovered). Recently a user glitched directly into there by wall glitching in the swamp, it's not shown in the video but he explored some of the grottoes and found a small area at the end of the video.