ATCO Atlas Corporation

As filed with the Securities and Exchange Commission on March 28, 2023

Registration Nos. 333-239578

333-263872

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Post-Effective Amendment No. 1

to Form S-8 Registration Statement No. 333-239578

Post-Effective Amendment No. 1

to Form S-8 Registration Statement No. 333-263872

UNDER

THE SECURITIES ACT OF 1933

23 Berkeley Square

London, United Kingdom

W1J 6HE

Telephone: +44 20 7788 7819

(Address, including zip code, and telephone number, including area code, of principal executive office)

Atlas Corp. Amended and Restated Stock Incentive Plan

(Full title of the plan)

Puglisi & Associates

850 Library Avenue

Suite 204

Newark, Delaware 19711

Telephone: (302) 738-6680

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

Hillary H. Holmes

Mark D. Director

Gibson, Dunn & Crutcher LLP

811 Main Street, Suite 3000

Houston, Texas 77002

Telephone: (346) 718-6600

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

DEREGISTRATION OF SECURITIES

These Post-Effective Amendments No. 1 (“Post-Effective Amendments No. 1”) to the Registration Statements on Form S-8, File Nos. 333-239578 and 333-263872 (collectively, the “Registration Statements”) are being filed by Atlas Corp., a Republic of the Marshall Islands corporation (“Atlas” or the “Registrant”), to deregister any and all common shares, par value $0.01 per share, of Atlas (the “Common Shares”) registered by the Registrant that remain unsold pursuant to the Atlas Corp. Amended and Restated Stock Incentive Plan.

Effective as of March 28, 2023, as contemplated by the Agreement and Plan of Merger, dated as of October 31, 2022, by and among the Registrant, Poseidon Acquisition Corp. (“Poseidon”), and Poseidon Merger Sub, Inc., a wholly-owned subsidiary of Poseidon (“Merger Sub”), Merger Sub merged with and into the Registrant with the Registrant continuing as the surviving corporation and a wholly-owned subsidiary of Poseidon (other than with respect to the Registrant’s preferred shares) (the “Merger”).

As part of the Merger, the Registrant has terminated any and all offerings of the Common Shares pursuant to existing registration statements, including the Registration Statements. In accordance with the undertakings made by the Registrant in these Registration Statements to remove from registration, by means of a post-effective amendment, any of the Registrant’s Common Shares that remain unsold at the termination of the offering, the Registrant hereby removes from registration, by means of these Post-Effective Amendments No. 1 to the Registration Statements, any and all Common Shares registered under these Registration Statements that remain unsold as of the effectiveness of the Merger on March 28, 2023.