Short asnwer is: "You can dig shells at Vigintia's Treasure Hunting Zone. during Tibia's Anniversary on special island Vigintia"
Longer answer is: Vigintia has a treasure cove to the south-west; ask Vigintius to go treasure hunting and he will charge you 150 gold for 90 seconds of digging. The 90 seconds begins when using the Closed Fence Gate to enter the treasure cove. You will be teleported out when your 90 seconds are up, but you can leave any time by using the Closed Fence Gate again. You can only dig with a Shovel, Light Shovels and Secret Agent Tools do not work
The items you can dig up are:
- Some Wood (You dig up some driftwood. But you are confident that you will find something precious soon.);
- Fishbone (You dig up a fishbone. But you are confident that you will find something precious soon.);
- Starfish (You dig up a lonely starfish. But you are confident that you will find something precious soon.);
- Shell (You dig up a sea shell. At least it might be tasty. But you are confident that you will find something precious soon.);
- Filthy Bunnyslippers (You have found bunnyslippers which were buried in the sand. But they look rather filthy.);
- Rusty Winged Helmet (You have found a winged helmet which was buried in the sand. But it looks rather rusty.);
- Tatty Dragon Scale Legs (You have found dragon scale legs which were buried in the sand. But they look rather tatty.);
- 3 White Pearls (You have found some pearls!);
- Coral Brooch (You have found a coral brooch!);
- Gold Nugget (You have found a gold nugget!).
- It's also possible to dig up nothing (You dig up nothing. But you are confident that you will find something precious soon.).
There is a limit of digging 101 times in 20 hours. If this limit is exceeded, the message (Your arms hurt from your efforts to find treasures on this beach. You should have a break. Return in some hours.) appears. If digging is attempted again before 20 hours pass, the attempt will fail with the message (Your arms still hurt from your efforts to find treasures on this beach. You should have a break. Return later.) Failing to dig until exhaustion will result in the number of digs never being reset. For this reason, it's recommended to enter the treasure cove twice or more until you receive the exhaustion message.
Source : Tibiawiki