This is a quick, safe and easy way to knock out a wild Mosasaurus by getting it stuck.
A wild Mosasaur will only swim up to a certain height. This height is the exactly where the location "Deep Ocean" changes to the location "Underwater". If it reaches this height, for example by chasing a player/tamed dinosaur or simply by swimming, it loses all aggro and tries to swim back into the "Deep Ocean". But since the Mosa is a big dino with a large turn radius, it is possible that it's momentum carries it into the location "Underwater" for a short amount of time.
While being in the location "Underwater", it will not attack, aggro or flee when receiving damage, it will only search for the quickest way to get back to the location "Deep Ocean". While searching, it will not swim further up and it will hardly change directions.
So, in theory, trying to get it stuck while it is in the location "Underwater" after it's momentum from chasing something carried it there might be a good idea because while being there, the wild Mosasaur poses no threat to players or tamed dinos.
Getting it stuck is a cheap, effective and safe alternative to using a taming pen, as keypads don't work underwater and a mosa can easily oneshot even a high-level player, which makes this method very useful for solo players or small tribes.
I recently found a nice spot to get a wild Mosasaur stuck. It is an alcove in one of the underwater-rocks near Herbievore Island on the map "The Island".
Edit: It's also possible to get a wild Mosa stuck in the rocks in the trench between Dead Island and the main land, but it's more difficult and i wouldn't suggest doing it without a strong water mount to block the wild Mosa in.
The alcove is exactly deep enough that a wild Mosa can be kited right next to it. When it hits the barrier between "Deep Ocean" and "Underwater", it will turn around and get right stuck in the alcove.
Note: I used my already tamed Mosa to block the wild Mosa in the alcove even further, but i think it also works with a Plesio, Megalodon or even a Dolphin (because it does not attack) or is not necessary in the first place. It's 100% possible with a tamed Mosa; i wasn't attacked once when i did it.
Som pictures to illustrate:
The Rock containing the alcove from a distance with herbievore Island in the background.
Picture from the setup with unconscious wild Mosa (red) and tamed Mosa (white, saddled).
Coordinates and location on minimap (unofficial server with no mods).
Edit: This method also works for killing Alpha Mosas. Just get it stuck to avoid getting attacked.
Picture is me attacking an Alpha Mosa I managed to get stuck in the rocks in the trench between Dead Island and the main land.
Alpha Mosa stuck in the rocks at Herbivore Island.
Written by Bolterwave.