Hi,

running this

> ensembl = useMart(biomart
> ="ENSEMBL_MART_ENSEMBL",dataset="hsapiens_gene_ensembl", host="may2017.archive.ensembl.org")
 
or this:

> ensembl = useMart(biomart
> ="ENSEMBL_MART_ENSEMBL",dataset="hsapiens_gene_ensembl", host="http://dec2017.archive.ensembl.org")

used to work in the past. but now it gives the error:

> Request to BioMart web service failed. Verify if you are still
> connected to the internet.  Alternatively the BioMart web service is
> temporarily down.  Check http://www.biomart.org and verify if this
> website is available. Error: XML content does not seem to be XML:

The site is up, the internet connection also.
In the site it says this:

> If you are a user of biomaRt (a part of the Bioconductor library)
> change the host from 'www.biomart.org' to 'www.ensembl.org'

But in the commands I used to run there's no 'www.biomart.org' to replace...

Do you know what is the correct command now?

I also tried:  

> ensembl =
> useMart(biomart="ENSEMBL_MART_ENSEMBL",dataset="hsapiens_gene_ensembl",
> host="www.ensembl.org")

and didn't work
As Emily has said, parts of Ensembl are down right now.  This includes the archive sites and the BioMart interface at the main site.  You can check this just by visiting in a browser (http://www.ensembl.org/biomart/martview?redirect=no) - at the moment you get an error page.

However you can use one of the mirror sites e.g. http://uswest.ensembl.org/biomart/martview?redirect=no

To do this using the **biomaRt** package you would do something like:

    ensembl = useMart(biomart="ENSEMBL_MART_ENSEMBL",
                      dataset="hsapiens_gene_ensembl", 
                      host="uswest.ensembl.org",
                      ensemblRedirect = FALSE)

I would also suggest updating your version of **biomaRt** (and maybe also R).  The error message about visiting www.biomart.org is misleading and is no longer produced in more recent versions of **biomaRt**.  You now get a more appropriate URL to try, that would have taken you to the error page at Ensembl.