Red Sea reefer 350 seam.

[wtrbug]

I had this tank running since 2014 without any issues. I noticed this week there is a problem on the bottom seal. It looks like a tunnel but I’m not sure how that can happen. There is water and substrate in this void. I’m waiting on a new tank and only noticed this because I was inspecting the tank before posting it for sale. Any ideas on what would cause this?

 

[ISpeakForTheSeas]

I don't know for sure, but possibly a worm burrowing into the silicone (it's rare, but it does happen):

With regards to the worm burrowing through silicone, I actually have seen this a couple of times before (see the quote below) - it is rare, but it seems it does happen (many worms are definitely capable of going through silicone, but few ever seem to do so):

Admittedly, my first instinct was something like ants too, but doing a little research, there might be something to the whole worm in the tank thing (the link has a video and picture, but you have to log in to view the picture):
https://www.ultimatereef.net/threads/bristle-worm-eating-through-my-aquarium-silicone-seals.888971/The worm in the video is tiny, but if you watch the line seemingly in the glass, you'll notice it move a little before the water starts spurting out.

Not saying it's confirmed that it's a worm, but it seems it could be a possibility.

Edit: just to add, as others have mentioned, it's entirely possible that the worm is just exploring/expanding an already existent defect in the silicone (i.e. it might not be causing the hole, just finding it and possibly adding to it).

https://www.reef2reef.com/threads/an-unidentified-worm-seems-to-be-eating-my-silicone.977474/