This page currently contains information that has never been confirmed. However, it is self-evident that it is true. See the "Other" section for more info on the speculation.
In 21 BBY, Malastare had a large fuel supply that both the Separatists and the Republic wanted it for themselves. The Republic sent a force of troopers and Jedi to stop the huge, advancing army of droids and to keep the fuel. Hawkeye was included in this group. The Republic also launched a new superweapon, an electro-proton bomb that would destroy mechanics, but not living people. It worked, but as a side effect, a giant sinkhole was created in the center of the droid army and the outside of the Republic army, bringing some troopers into it.
The Republic sent a group of people to see what happened, and they never came up. Hawkeye, Jedi General Mace Windu, Trapper, Ponds, and some others went down to investigate. They landed on what seemed a hard surface and looked around. Soon, it was revealed that the "hard surface" was actually the back of a giant and ancient creature called the Zillo Beast. The team barely escaped with the help of Anakin Skywalker however, Hawkeye was killed.