swan dive

Short definition from Wiktionary

Noun

1. 1
   A forward dive performed with an arched back, the legs together, starting with arms outstretched when jumping, bringing them over the head when landing.
2. 2
   A similar movement out of the water.
3. 3
   A sharp drop or steep decline.

Verb

1. 1
   to perform a swan dive
2. 2
   to plummet, drop rapidly