The Depths is one of the three maps/areas that Link can explore in Tears of the Kingdom. While most players anticipated the Sky Islands thanks to teaser trailers, The Depths took some by surprise. It's only natural that there would be dangerous and new monsters lurking below the surface of Hyrule.
Players that lend a hand to Tears of the Kingdom's Monster-Control Crew will eventually get the side quest "Unknown Huge Silhouette." Link will need to find and defeat a monster called a Frox. It lives somewhere in The Depths, so players will want to make sure they have plenty of Brightbloom Seeds to light the way.
How to Unlock "Unknown Huge Silhouette"
The "Unknown Huge Silhouette" and two other side quests ("Unknown Sky Giant" and "Unknown Three-Headed Monster") are locked behind three prerequisites. Link will need to speak to Gralens in the Lookout Landing Shelter. Gralens is a member of the Monster-Control Crew in Tears of the Kingdom. He'll be standing over a table next to a map of Hyrule.
Ask about the giant monsters to get the following side quests:
- WANTED: Hinox
- WANTED: Stone Talus
- WANTED: Molduga
The locations of these monsters can be tracked from the Adventure Log. Open the map with (-), then tab over to Adventure Log and select one of the WANTED side quests. This will place a yellow destination marker, making it easier for players to complete the three quests.
"Unknown Huge Silhouette" Walkthrough
With the "WANTED" side quests taken care of, speak to Gralens again and inquire about the new monsters. Select "the huge silhouette" to get the "Unknown Huge Silhouette" side quest. Gralens will then mark the location of the monster on Link's map, which happens to be hiding in The Depths.
Before going to the location, players will want to make sure they have a weapon that's good for breaking ore deposits. It's also recommended to have 
Bomb Flowers, too. The monster is near two Lightroots, but players may want to bring some 
Brightbloom Seeds just to be safe.
If players defeat the Stone Talus from the "WANTED: Stone Talus" quest, they can fuse the Talus Heart to one of their weapons. A Talus Heart deals "critical" damage to ore deposits.
"Unknown Huge Silhouette" Location
Players will find the monster a little northwest of the Stakijat Lightroot (0343, -1008, -0468). A good way to reach this area is to enter the Hyrule Field Chasm and go southeast. Once players near the yellow marker, they'll see a Frox, which is the "silhouette" mentioned in the side quest.
Frox can be defeated by breaking the ore deposits on its back. They're slow-moving, but they can slam into the ground and hit Link with a shockwave. They will also inhale anything that gets close. Players can prevent being dragged into the Frox's mouth by sprinting.
Once the Frox is defeated, collect whatever it drops and then warp back to Lookout Landing.
Tips for Defeating Frox
Like the Hinox, a Frox can be stunned by shooting its eye. This will cause it to drop and allow Link to climb up its limbs and onto its back. Equip a deposit-breaking weapon (ex: Stone Talus Heart fused to a sword/lance/two-handed weapon) and start hitting the deposits. When one breaks, move to the other.
Once the Frox recovers, it'll launch Link into the air if he's still on its back. If it does this, open the paraglider with X-button and maneuver Link over the Frox's head. Fire an arrow at its eye to stun it again.
Alternatively, the Frox may open its mouth. If this happens, fuse a bomb flower to an arrow and fire it into the Frox's mouth. The Frox will eat the bomb, which will stun it and give Link some time to climb up its front legs and onto its back again.
"Unknown Huge Silhouette" Reward
Report to Gralens and select the first option. This will mark the "Unknown Huge Silhouette" as complete, and Gralens will give Link 100 Rupees for a reward. If, by some chance, players have already defeated the Frox near the Stakijat Lightroot, they will receive the 100 Rupee reward after they inquire about the "huge silhouette."
The Legend of Zelda: Tears of the Kingdom is available for Nintendo Switch.