SEC Form 3

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person* Muller Elizabeth (Last) (First) (Middle) C/O DEEP FISSION, INC. 2831 GARBER STREET (Street) BERKELEY CA 94705 (City) (State) (Zip)	2. Date of Event Requiring Statement (Month/Day/Year) 09/05/2025	3. Issuer Name and Ticker or Trading Symbol DEEP FISSION, INC. [ NONE ]
		4. Relationship of Reporting Person(s) to Issuer (Check all applicable) X Director 10% Owner X Officer (give title below) Other (specify below) President and Chief Executive	5. If Amendment, Date of Original Filed (Month/Day/Year)
			6. Individual or Joint/Group Filing (Check Applicable Line) X Form filed by One Reporting Person Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Employee Stock Option (right to buy)(1)	(2)	08/28/2035	Common Stock	811,699	$51.2	D

Explanation of Responses:

1. Received pursuant to the terms of the Agreement and Plan of Merger and Reorganization dated as of September 5, 2025, by and among the Issuer (f/k/a Surfside Acquisition Inc.), Deep Fission Acquisition Co. and Deep Fission Inc. ("Legacy DF") in exchange for options to acquire shares of Legacy DF common stock. The number of shares of Legacy DF common stock subject to such options (the "option shares") was converted into an option to purchase the number of shares of the Issuer's common stock equal to the option shares multiplied by 17.32142 (the "Conversion Ratio"), rounded down to the nearest whole share, and the exercise price of each option was divided by the Conversion Ratio, rounded up to the nearest whole cent. The options were acquired by the Reporting Person prior to the Reporting Person becoming an executive officer of the Issuer. The Reporting Person was appointed as a director and executive officer of the Issuer effective immediately after the effective time of the Merger.

2. The shares subject to this option vest over a 4-year period, with 25% of the underlying shares vesting on June 1, 2025 and 1/48th of the shares underlying shares vesting monthly over the 36-month period thereafter, subject to the executive's continued employment through each vesting date.

	/s/ Elizabeth Muller	09/09/2025
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
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