{raak} No

[Darren] I don't know, you'll have to ask them ;-)

(No - not as such - more just a general sense that the question could lead in the right sort of direction)

[Iroul] No
(won't be responding for a few hours now)

[Inks] No
(I did debate with myself whether or not to answer Yes to the geology question)

[Darren] No

[Inks] No
[CdM] No
[Iroul] No
This is fun, isn't it?

[Flerdle] No, not as such, but that's not a bad line of thought to pursue
I'll be available to answer questions until for about another 4 hours - after that I will probably be offline until Monday evening or Tuesday, so I'll start putting clues out in a couple of hours if no-one's got it by then

[Chalky] No
[Darren] No

[Flerdle] No
[Iroul] No

OK - to summarise those things which might help:
It is found on earth (though earth is not part of the answer - which is more than one word), it is broadly to do with geology, it is part of mainstream science, it cannot be perceived without instruments, it doesn't exist on the moon and not as such on other planets, it is not an event nor anything to do with weather (though that might have got the audience murmuring as well).
You've been concentrating a lot on what - there may be other productive lines of enquiry.

[Flerdle] No (but that's partly because of the specific description)

[Rosie] No - not quite (but loud and prolonged applause from the audience)

[Flerdle] No (the audience are on the edge of their seats)

Oh actually you can perceive that without instruments so ignore that one.

Being a bit less clever, this time! Heh.

(since Darren took the other pole, and the tendency here is to be pedantic, I take it)

[Darren] No
Flerdle YES (Thunderous applause, with maybe just a slight tinge of relief, from the audience)
I might have given it to you anyway if Rosie hadn't already said 'the magnetic field'.

So, it's over to Flerdle

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Oh. Ah. umm... M I N E R A L