The Sea Snake's Slithering Problem

Question

Here's a really long chess puzzle, not made by me, that has the "sea snake" theme, which just basically means that it's super long.

It's White to move and checkmate Black in 122 moves.

Миљанић, Мирко, MatPlus, 12/1996

Please do not look up the solution nor your computer at all! You must use your brain power alone to solve this

This is a challenge meant to make you really think!

Answer 1

Observation.

It doesn't seem possible to find a sequence of checks that achieves anything, however there is a sequence of checkmate threats that forces Black to defend in only one possible way at each time.

Let us start with

1. f8=Q+!!

The next move must be

1. ... Kb2
because otherwise
1. ... Ka4 2. Qxe8+ Ka3 3. Qb5 Rb1 4. Qxa5+ Kb2 5. Qc3#

Then

2. Qf2+ must be met with 2. ... Kb1
otherwise 2. ... Ka3 3. Qb6 and the same mating pattern repeats.

After this, 3. Kc3 comes again with a deadly checkmate threat. If 3. ... Ba4 then there is mate in 3 with 4. Qb6. Hence the only move is 3. ... Bd2+ 4. Qxd2.
There are two threats and the only Black move is 4. ... Rc1+.
Then after 5. Kb3, 5. ... a4 doesn't work because of 6. Ka3 and Black can't prevent mate in 2. Rook moves clearly don't work either. In fact, 5. ... Ba4 will work (and is the only move) because it now covers the c2 square. 6. Ka3 (you want to keep the pressure) forces 6. ... Rc2.

From now on the strategy is to

get your Queen to e4, with all other pieces where they are now, and with Black to play. From e4 the Rook is pinned, the h1 square is under control (in case Black wants to make a Queen for themselves), and the e1 square is under control (in case the Black King would like to run away, see below), and most importantly, after a futile Bishop maneuver, the only move for Black that doesn't lose instantly is to push a pawn. The sequence that allows this is 7. Qd1+ Rc1 8. Qd3+ Rc2 9. Qe4.
Note that 9. ... Kc1 loses instantly to 10. Qe1#,
and 9. ... Bb3 achieves nothing due to 10. Kxb3 a4+ 11. Ka3.

So, as planned

9. ... h1=Q. We need to get rid of this Queen and iterate.
10. Qxh1+ Rc1 11. Qe4+ Rc2 12. Qe1+ Rc1 13. Qd2 Rc2 14. Qd1+ Rc1 15. Qd3+ Rc2 16. Qe4

16. ... h3 (Black pees in their pants)

Here since we don't need to capture anything at h1, the iteration is faster:
17. Qe1+ Rc1 18. Qd2 Rc2 19. Qd1+ Rc1 20. Qd3+ Rc2 21. Qe4

Let's fast forward:

It takes 5 moves to push one pawn without promotion, and 7 moves to promote a promotable pawn. Therefore, in $4\times 7 + 13\times 5=93$ moves, we will have 114. Qe4
with no more pawns in the h file.
Then after 114. ... e5 and 5 more moves, 119. Qe4 where now the only way to delay more is the Bishop maneuver: 119. ... Bb3 120. Kxb3 a4+ 121. Ka3.

From where the only allowed move is 121. ... Kc1, and the final blow 122. Qe1#.