MY brother-in-law used to live in a house in Los Angeles with the San Andreas fault literally at the bottom of his garden.

You could look out of the upstairs windows and see a distinct line where the ground had fractured.

Under his stairs he kept an emergency store, just in case an earthquake hit.

He would probably think I was bonkers to spend the best part of an afternoon looking at a large crack in an art gallery floor.

But that is exactly what I did when I visited London last week.

For me, an autumn trip to the capital isn't complete without a visit to the Tate Modern to see the latest spectacle in the Turbine Hall.

Since Louise Bourgeois' 10-metre-high spider lurked menacingly above the ticket offices eight years ago, the installations have maintained their curiosity value.

Crowds of visitors have flocked to the capital's modern art gallery to see such creations as Olafur Eliasson's giant sun bathing the stark space with chilly, golden light; a sculptural landscape constructed out of white plastic boxes by Rachel Whiteread; and, last year, Carsten Höller's giant flumes, which allowed the intrepid to experience whizzing down massive tubes of steel from the very top of the building.

One year the space was left completely empty - physically at least - but filled with sounds in the form of strange voices and whispers by Bruce Nauman.

This year, of course, finds Bourgeois' giant arachnid back, but this time standing guard outside the former power station building on the banks of the Thames.

It is within that you find Doris Salcedo's Shibboleth, the much-publicised crack which cuts through the floor of the 180-metre-long exhibition space, giving the impression that England's capital city has been victim of an earthquake all of its own and that a huge fault line has opened up down the full length of the building.

It is an extraordinary sight. Starting as nothing more that a hairline fissure at the west entrance, the fracture gradually opens up to reveal a trench which, while it is only about a metre deep and is not particularly wide, is intriguingly ambiguous in its look.

Frankly, at some points the rift looks as if it could go down and down for ever.

It is fascinating to watch people interact with it. The initial approach seems to be cautious, possibly because on the first day it was on public display, a few careless viewers actually managed to fall into it.

Children seem to have the most fun. Several I saw were happily engrossed stepping or jumping over it, peering into it, and generally marveling at the scale of this bizarre spectacle. Adults too seemed entranced, some lying on the floor to examine this visual statement properly.

Whatever your views of modern art, this spectacle is worth a visit. So if you're in London, do go.

It's quite safe, and unlike Southern Californians, you won't need to have emergency rations on hand - the Tate's restaurants are really quite good.