From the end of chapter 16:

The Mungana shook his head and began to enter the canal. Jeekie, whose
teeth were chattering, hung back, but Alan pushed him from behind, so
sharply that he stumbled and made a splash. Then Alan followed, and as
the cold, black water rose to his chest, looked again at Big Bonsa.

It seemed to him that the thing had turned round and was staring at
them. Surely a few seconds ago its snout pointed the other way. No, that
must be fancy. He was swimming now, they were all swimming, Alan and
Jeekie holding their pistols and little stock of cartridges above
their heads to keep them dry. The gold head of Big Bonsa appeared to be
lifting itself up in the water, as a reptile might, in order to get a
better view of these proceedings, but doubtless it was the ripples that
they caused which gave it this appearance. Only why did the ripples make
it come towards them, quite gently, like an investigating fish?

It was about ten yards off and they were in the middle of the canal. The
Mungana had passed it. It was in a line with Alan's head. Oh Heavens! a
sudden smother of foam, a rush like that of a torpedo, and set low down
between two curving waves, a flash of gold. Then a gurgling, inhuman
laugh and a weight upon his back. Down went Alan, down and down!

CHAPTER XVII

THE END OF THE MUNGANA

The moonlight above vanished. Alan was alone in the depths with this
devil, or whatever it might be. He could feel hands and feet gripping
and treading on him, but they did not seem to be human, for there were
too many of them. Also they were very cold. He gave himself up for dead
and thought of Barbara.

Then something flashed into his mind. In his hand he still held the
revolver. He pressed it upwards against the thing that was smothering
him, and pulled the trigger. Again he pulled it, and again, for it was a
self-cocking weapon, and even there deep down in the water he heard the
thud of the explosion of the damp-proof copper cartridges. His lungs
were bursting, his senses reeled, only enough of them remained to tell
him that he was free of that strangling grip and floating upwards. His
head rose above the surface, and through the mouth of his mask he drew
in the sweet air with quick gasps. Down below him in the clear water
he saw the yellow head of Big Bonsa rocking and quivering like a great
reflected mon, saw too that it was beginning to rise. Yet he could not
swim away from it, the fetish seemed to have hypnotized him. He heard
Jeekie calling to him from the shallow water near the further bank, but
still he floated there like a log and stared down at Big Bonsa wallowing
beneath.

Jeekie plunged back into the canal and with a few strong strokes reached
him, gripped him by the arm and began to tow him to the shore. Before
they came there Big Bonsa rose like a huge fish and tried to follow
them, but could not, or so it seemed. At any rate it only whirled round
and round upon the surface, while from it poured a white fluid that
turned the black water to the hue of milk. Then it began to scream,
making a thin and dreadful sound more like that of an infant in pain
than anything they had ever heard, a very sickening sound that Alan
never could forget. He staggered to the bank and stood staring at it
where it bled, rolled and shrieked, but because of the milky foam could
make nothing out in that light.

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See Thread II (click)