Hydra is accessed via an ssh client like MobaXTerm, Putty or any native 'ssh' command.
The cluster lives behind a firewall to protect it from attacks from the whole internet. The best way through this firewall is to connect via the Physics ssh gateway server, bastion.physics.ox.ac.uk and use it as a "jump host" or "ssh gateway".
Instructions for this are below. Please note that bastion will ask you for your Physics username/password and Hydra will ask you for your Hydra username/password - they are different.

OpenSSH (Linux, macOS, Windows OpenSSH)

Modern SSH clients support jump hosts natively.

Edit:

~/.ssh/config

and add:

Host bastion
    HostName bastion.physics.ox.ac.uk
    User PHYSICS_USERNAME
    ForwardX11 yes
    ForwardX11Trusted yes

Host hydra
    HostName hydra.physics.ox.ac.uk
    User HYDRA_USERNAME
    ProxyJump bastion
    ForwardX11 yes
    ForwardX11Trusted yes

Replace:

• PHYSICS_USERNAME with your Physics Department username
• GLAMDRING_USERNAME with your Hydra username

Save the file and ensure permissions are correct:
chmod 600 ~/.ssh/config

Then you can simply "ssh hydra" to connect from anywhere.