The opening is percussive with key clicks and tones produced by striking the mouthpiece. The rhythms of the key clicks are the same as used by playing with tones later. The main motive is the trill - which on a tuba can't be played without key clicks. The central episode is meant to describe both diving beneath the ocean and dreaming. After rising to the surface (half-forgetting the dream) the tuba launches into a development of the themes presented before the dive. The climax is a loud trill which suddenly drops the tone and becomes simply key clicks (a mechanistic skeleton).
Zacariah Spellman is an amazing player who adapts the digeredoo technique of circular breathing to the tuba. He can keep a tone going indefinitely.
Tubessence text (recited into the tuba while playing):
Under the waves
It's all here
But you can't stay here for long
Something reminds you to leave
And you half forget
(for solo tuba) © 2000, Steve Mobia