Theme: "Code Crackers" - The letters, as hinted by part of the theme entries, orderly spelled out FBI AGENTS.
23A. One way to enter a pool: FEET FIRST. The first letter of the word "Feet" is F.
29A. One end of a church key: BOTTLE OPENER. The opening letter of "Bottle" is B.
36A. Physical location?: MEDICAL CENTER. The very center of "Medical" is I.
54A. "Mobile" communications device used in law enforcement: DATA TERMINAL. The terminal letter of "Data" is A.
64A. Tchaikovsky's "1812 Overture" has one that includes cannon fire: BIG FINISH. The finishing letter of "Big" is G.
72A. 2012 Beyoncé hit with the repeated line "Say you'll never let me go": END OF TIME. The ending letter of "Time" is E.
81A. Summing-up words: IN CONCLUSION. The concluding letter of "In" is N.
102A. Guru whose opinions are trusted: THOUGHT LEADER. The leading letter of "Thought" is T.
110A. 2019 "Game of Thrones" event: SERIES FINALE.The final letter of "Series" is S.
Reveal entry:
120A. Govt. employees encoded by the nine other longest across entries in this puzzle ... and who might be called in to decode them: FBI AGENTS.
Fun gimmick from Mark.
Reminds me of the "DEAR JOHN LETTERS" puzzle I made of for the NYT a while ago. So I fully understood the amount of research and work he put into this grid.
Across:
1. College loan co-signer, maybe: DAD. None for me, thankfully. Chinese government paid all my college education.
4. Establish as law: ENACT.
9. Long-legged birds: WADERS.
15. Five-O nickname: DANO.
19. Before, poetically: ERE.
20. Prepare to surf: LOG ON. Most Chinese use WeChat. Quite a few neat features.
21. Reunion attendee: ALUMNA.
22. King Harald's father: OLAV.
25. __ media: SOCIAL.
26. Film scene shot without interruption: TAKE.
27. Kenya's first prime minister __ Kenyatta: JOMO. No idea. Wiki says he was also Kenya's first president. He died in office in 1978.
28. Hydroelectric project: DAM.
31. Amazon assistant: ALEXA.
33. Abundant element in Earth's core: IRON.
35. Thingamajigs: DOODADS.
41. Humor: INDULGE. me.
42. Winans of gospel: CECE.
43. Most hip: COOLEST.
45. Supposed to arrive: DUE.
46. Spa sounds: AHS. And 49. Plotting chuckle: HEH.
51. Delta rival renamed in 1997: US AIR.
53. Moving aids: VANS.
60. Jack in a suit: KNAVE.
62. Initial Hebrew letter: ALEPH. We also had 39. 12th Jewish month: ELUL.
63. Mountain melody: YODEL.
68. Lose it: GO APE.
69. Matching game: LOTTO. Our old neighbor won $100,000. Lost them all, including his house.
71. Total, as a bill: RUN TO.
76. Slips through the cracks: OOZES.
79. Domingo, for one: TENOR.
80. One of Australia's six: STATE.
83. Thin coin: DIME.
86. Digitally approve: E-SIGN.
88. Game-winning line: OOO.
89. Hallelujah trio?: ELS. Halleluja. Just the letters.
90. Spanish article: UNA.
91. Sharp tingle, as of fear: PRICKLE.
95. Ford contemporary: OLDS.
98. Discount: CUT RATE.
107. Surname on Elm Street: KRUEGER (Freddy)
108. Chiwere-speaking native: OTOE.
109. Trial subject: CRIME. Even our local Star Tribute carried this story.
114. 45, in classic pop: RPM.
117. Well-behaved: GOOD.
118. Like many Horace works: ODIC.
119. Mexican madam: SENORA.
122. __ Reader: UTNE.
123. When Hamlet kills Polonius: ACT III.
124. Divider of pews: AISLE.
125. Word in many obituaries: NEE.
126. Collectible '90s caps: POGS.
127. Put away for later: STORED.
128. Kingdom: REALM.
129. Important stretch: ERA.
Down:
1. Kanye West label: DEF JAM.
2. Ring of color: AREOLE.
3. Regarded: DEEMED.
4. 2003 holiday comedy: ELF.
5. Reason for being barred at a bar: NO ID.
6. Taj Mahal city: AGRA.
7. Immense: COSMIC.
8. Blast cause: TNT.
9. Suspected: WAS ONTO.
10. Heaps: A LOT.
11. Tear channel: DUCT.
12. Czech track legend Zátopek: EMIL.
13. Cellular process affecting nucleotide sequences: RNA EDITING. New term to me.
14. Scene of some "Gunsmoke" action: SALOON.
15. College address ending: DOT EDU.
16. Actor born Alphonso D'Abruzzo: ALAN ALDA. Full name.
17. First of three Leslie Nielsen comedies, with "The": NAKED GUN.
18. Supervises: OVERSEES.
24. Venomous: TOXIC.
29. U2 frontman: BONO.
30. Whale group: POD.
32. Pitching staff star: ACE. Life is so fragile. You never know what's going on in other's life.
34. VCR button: REC.
37. Masseuse's target: ACHE.
38. Not sure (of): LEERY.
40. Hi-__ graphics: RES.
44. Munro pen name: SAKI. I wonder why he picked this name.
46. "Great minds think alike," e.g.: ADAGE.
47. Fire-suppressing gas: HALON.
48. Position: STEAD.
50. Med. care provider: HMO.
52. U.K. fliers: RAF.
53. They may be changed by judges: VENUES. Trial venue.
55. Place side by side: APPOSE.
56. Taking it badly?: THEFT. Fun clue.
57. Run in place: IDLE.
58. Modernist's prefix: NEO.
59. TV chef Brown: ALTON.
61. Love of antiques: VIRTU.
64. Renowned clown: BOZO.
65. Navel formation: INNIE.
66. Stand-up comic's seat: STOOL.
67. Lock __: come into conflict: HORNS.
70. Front-of-bk. list: TOC. Table of Content.
73. __ Bo: TAE.
74. "Just what I wanted!": IT'S PERFECT.
75. '70s Israeli prime minister: MEIR.
77. "Music for Airports" producer: ENO.
78. Move quickly: SCOOT.
81. Ruler unit: INCH.
82. Veg out: LOLL.
83. A piece of cake: DUCK SOUP. Love this fill.
84. More than familiar with: INURED TO.
85. Growing up: MATURING.
87. "Shoo!": GIT.
92. Brand with a pitcher-shaped mascot: KOOL-AID.
93. Bard's instrument: LUTE.
94. Freudian focus: EGO.
96. Mo. when Festivus is celebrated: DEC.
97. Beetle Bailey nemesis: SARGE.
99. Treats again, as a sprain: RE-ICES. Or hernia.
100. Grow older: AGE.
101. Actress Thompson and ice dancer Virtue: TESSAS.
103. Disney Beetle: HERBIE. Herbie the Love Bug.
104. "I'll Never Fall in Love Again" singer Warwick: DIONNE.
105. Drama queen, e.g.: EMOTER.
106. View from Jidda: RED SEA.
111. Really digging: INTO.
112. Pinot __: NOIR.
113. Grammy winner India.__: ARIE.
115. Galileo's birthplace: PISA. Oh hello!
116. Strip __: MALL.
120. Away companion: FAR.
121. Rock in a setting: GEM.
Belated
happy birthday to dear CanadianEh, one of the most insightful
commenters on our blog. How did you celebrate this year, Eh?