Construction in progress is primarily comprised of construction of Gigafactory Berlin-Brandenburg and Gigafactory Texas, expansion of Gigafactory Shanghai and equipment and tooling related to the manufacturing of our products. Completed assets are transferred to their respective asset classes and depreciation begins when an asset is ready for its intended use. Interest on outstanding debt is capitalized during periods of significant capital asset construction and amortized over the useful lives of the related assets. During the three and nine months ended September 30, 2022, we capitalized interest of an immaterial amount. During the three and nine months ended September 30, 2021, we capitalized $14 million and $52 million, respectively, of interest.
Depreciation expense during the three and nine months ended September 30, 2022 was $620 million and $1.75 billion, respectively. Depreciation expense during the three and nine months ended September 30, 2021 was $495 million and $1.38 billion, respectively. Gross property, plant and equipment under finance leases as of September 30, 2022 and December 31, 2021 was $2.76 billion and $2.75 billion, respectively, with accumulated depreciation of $1.56 billion and $1.21 billion, respectively.