	(in thousands)
December 31,	2021			2020
Land and improvements, net	$	845,772		$	744,305
Buildings and improvements		1,845,937			1,505,913
Machinery and equipment		13,119,177			12,204,738
Proved oil and gas properties		558,336			558,231
Leasehold interest in unproved oil and gas properties		96,000			138,000
Construction in process and equipment deposits		2,039,245			1,603,416
		18,504,467			16,754,603
Less accumulated depreciation		(10,389,649	)		(9,855,493	)
	$	8,114,818		$	6,899,110

The estimated useful lives primarily range from five to 25 years for land improvements, four to 40 years for buildings and improvements and two to 15 years for machinery and equipment. The useful life for proved oil and gas properties is based on the unit-of-production method and varies by well.

In 2019, Nucor recorded a non-cash impairment charge of $20.0 million related to certain property, plant and equipment at our plate mill in Texas. This charge is included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2019.

In 2020, Nucor recorded non-cash impairment charges totaling $103.2 million related to certain inventory and long-lived assets, which primarily related to our Castrip sheet mill operations. Due to the advancements in the capabilities at our new cold mill and galvanizing line at Nucor Steel Arkansas, we believe the value of the technology and process has diminished for Nucor. As such, the existing Castrip assets are not expected to be materially utilized going forward. These charges are included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2020.

In the fourth quarter of 2019, due to the deteriorating natural gas pricing environment at our sales point in the Piceance Basin as well as the decreased performance of its natural gas well assets, Nucor determined a triggering event had occurred and performed an impairment analysis on all three groups (“fields”) of wells. As a result of the fourth quarter of 2019 analysis, a $35.0 million non-cash impairment charge was recorded on one field of wells. An increase in the estimated lease operating cost projections was the primary factor in causing this field of wells to be impaired. The non-cash impairment charge is included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2019.

One of the primary assumptions that affects the undiscounted cash flows determination is management’s estimate of future pricing of natural gas and natural gas liquids. The pricing used in the impairment assessments was developed by management based on projected natural gas market supply and demand dynamics, in conjunction with a review of projections by market analysts. Management also makes estimates on the expected reserve levels and on the expected lease operating costs. The impairment assessments were performed on each of Nucor’s three fields of wells, with each field defined by common geographic location. The combined carrying value of the three fields of wells was $65.2 million at December 31, 2021 ($71.7 million at December 31, 2020).

Changes in the natural gas industry or a prolonged low-price environment beyond what had already been assumed in the assessments could cause management to revise the natural gas and natural gas liquids price assumptions, the estimated reserves or the estimated lease operating costs. Unfavorable revisions to these assumptions or estimates could possibly result in further impairment of some or all of the fields of proved well assets.

In 2020, regulatory authorities in Colorado adopted new rules that became effective January 2021. One of these rules increased drilling setback distances. In the fourth quarter of 2020, Nucor determined a triggering event had occurred, as we do not expect to be able to access the full extent of the resources in the ground, and performed an impairment analysis. As a result, Nucor recorded a $27.0 million non-cash impairment charge related to the write-down of our leasehold interest in unproved oil and gas properties. This charge is included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2020.

In the second quarter of 2021, Nucor decided that it would not develop a portion of its unproved oil and natural gas properties (“Portion A”) within the contractually specified time period related to Portion A. As a result of this decision, the Company will forfeit its leasehold rights for Portion A. The Company recorded a charge of $42.0 million to write off the value of Portion A that is included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2021. The decision not to develop Portion A was heavily influenced by the approaching deadline to commence development combined with Portion A’s expected near-term profitability not achieving management’s desired returns relative to the cost of development. A significant portion of the Company’s remaining leasehold interest in unproved oil and natural gas properties are held by production. Accordingly, management does not believe the value assigned to those portions needs to be evaluated at this time. The carrying value of the remaining portions of unproved oil and natural gas properties was $96.0 million at December 31, 2021.