My girlfriend and I just spenmt some time in Acapulco and we were
looking out over the main bay area  there and being the geeks we are,
were wondering how did it form? In specific, it's clear that there's
been vulcanic activity in the area and we were wondering if the near
circular shape of the bay was caused by the colapse of an ancient
caldera/vulcano?

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Request for Question Clarification by techtor-ga on 30 Jul 2003 07:00 PDT

There are several theories about the formation of Acapulco bay, nine
of which have been decided upon as final. Would you accept a list of
these theories as an answer?

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Clarification of Question by tcp-ga on 30 Jul 2003 07:33 PDT

A listing of the nine theories and any pointers to further arguments
would certainly be sufficient to answer this question!  Thank you!

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Clarification of Question by tcp-ga on 30 Jul 2003 07:34 PDT

how I translated several into "nine", I have no clue...that's what I
get for trying to work before having my coffee.  ack.  that along with
my typos....

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Request for Question Clarification by techtor-ga on 31 Jul 2003 09:27 PDT

My pardon... I myself mistyped 'none' as 'nine' in my clarification!
Apologies for the error. Actually, only two theories were mentioned so
far in my searches... I hope that can do for you.

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Clarification of Question by tcp-ga on 31 Jul 2003 10:39 PDT

However many theories (well, reasonable theories) there are out there,
I'd be glad to see reference to all/any of them!  Thanks!

I hope this scant information I can provide you will still be worth
your $5. Two theories are provided, but I believe no conclusion has
been made on them.

The Encyclopedia Of Quantavolution And Catastrophe By Alfred De Grazia
http://www.grazian-archive.com/quantavolution/Encyclopedia/encqnt1.htm
- "A circular depression like Acapulco Bay is a possible astrobleme or
it could be a fossil caldera of an old volcano. Subsurface probing
would be required to validate these or other mechanisms for the bay's
formation."

I was unable to find any more details about these theories or any
other theories online.

Note: An astrobleme is "a scar on the earth's surface left from the
impact of a meteorite." (dictionary.com)

I can mention a bit of other geographic information about Acapulco
Bay:

Tsunamis article 
http://www.johnmartin.com/earthquakes/eqpapers/00000051.htm
- mentions that Acapulco is near a trench where seismic activity is
common, and this activity causes tsunamis. I've also seen in the above
source that the region Acapulco is in experiences earthquakes often,
quite like California.

Travel Guide Destination Mexico
http://www.packback.com/books/country/Mexico.htm
- The Southern regions of Mexico are full of volcanic mountains, and
Acapulco is situated among these.

Here is a simple map of Acapulco bay for reference:
http://www.lonelyplanet.com/mapshells/north_america/acapulco/acapulco.htm

Google Search:
acapulco bay formation
acapulco bay volcanic
acapulco bay seismic activity

I hope this has been a helpful answer. If you have any problem with
it, do please post a Request for Clarification and I shall respond as
soon as I can. Thank you.

Honestly, this is as much of an answer as I think I could expect for
$5.00.  You've shown that there isn't an easy/clear answer out there
that I was missing and you've also pointed me to some information. 
All in all, it's a quality answer.  Thanks for the work.