Here are a few contenders:

* The Earth's core: If you're talking about volume, the Earth's core is the largest single object on Earth. It's a massive ball of solid iron and nickel, around 1,220 kilometers (758 miles) thick.

* The Great Barrier Reef: If you're thinking about the largest living structure, then the Great Barrier Reef wins. It's a collection of over 2,900 individual reefs and 900 islands stretching over 2,300 kilometers (1,400 miles).

* The Greenland Ice Sheet: If you're considering surface area, the Greenland Ice Sheet is the largest. It covers an area of 1,710,000 square kilometers (660,000 square miles).

* The Amazon Rainforest: If you're talking about a continuous ecosystem, the Amazon Rainforest is the largest. It spans over 5,500,000 square kilometers (2,100,000 square miles).

So, the answer really depends on what criteria you use to define "biggest"!