Deadfish~, 3↑(2↑↑200)~~2↑4000
iii{{ss{iss}}}o
Crashes TIO, but in theory works.
This means:
3222...2Repeat 1000 times: add one, than square twice.
With 200 2sFirst iteration: 0 => 1 => 1 => 1 Second iteration: 1 => 2 => 4 => 16 Third iteration: 16 => 17 => 289 => 83521 And after this it grows without bound.