pre open sales

Hi guys, I have a question about pre open sales, it applies to all shares but as I am following BHP and have noticed some strage trades I will ask the question here on the BHP forum.

I see trades go through for very unsual amounts, over the last week or so I have seen shares go through at $43 when the market is only around $ 32 to $34, and today 100 000 shares trade at $13.446 but then a minute later the same 100 000 is traded with the (-) minus sign at $13.446.

A number of times now I have seen trades be placed only to be retracted a few minutes later, not sure if retracted is the right term, maybe canceled.

I had assumed these pre open large trades were made by big institutions, blue chip managed funds, but the unusual high and low prices have me a little confused, as do the (-) negative values being traded, I thought maybe these trades where options being exercised ?

Thanks for any help with this one.

Tony.